Still, the White House signaled that Obama did plan to step up U.S. involvement in the Syrian crisis in response to the chemical weapons disclosure.
"This is going to be different in both scope and scale in terms of what we are providing," said Ben Rhodes, Obama's deputy national security adviser.
The U.S. has so far provided the Syrian rebel army with rations and medical supplies.
Thursday's announcement followed a series of urgent meetings at the White House this week that revealed deep divisions within the administration over U.S. involvement in Syria's civil war. The proponents of more aggressive action including Secretary of State John Kerry appeared to have won out over those wary of sending weapons and ammunition into a war zone where Hezbollah and Iranian fighters are backing Assad's armed forces, and al-Qaida-linked extremists back the rebellion.
Obama still opposes putting American troops on the ground in Syria and the U.S. has made no decision on operating a no-fly zone over Syria, Rhodes said.
U.S. officials said the administration could provide the rebels with a range of weapons, including small arms, ammunition, assault rifles and a variety of anti-tank weaponry such as shoulder-fired remote-propelled grenades and other missiles. However, a final decision on the inventory has not been made, the officials said.
Most of those would be weapons the opposition forces could easily use and not require much additional training to operate. Obama's opposition to deploying American troops to Syria makes it difficult to provide much large-scale training. Other smaller- scale training can be done outside Syria's borders.
All of the officials insisted on anonymity in order to discuss internal administration discussions.
Word of the stepped up assistance followed new U.S. intelligence assessments showing that Assad has used chemical weapons, including sarin, on a small scale multiple times in the last year. Up to 150 people have been killed in those attacks, the White House said, constituting a small percentage of the 93,000 people killed in Syria over the last two years.
The White House said it believes Assad's regime still maintains control of Syria's chemical weapons stockpiles and does not see any evidence that rebel forces have launched attacks using the deadly agents.
The Obama administration announced in April that it had "varying degrees of confidence" that sarin had been used in Syria. But they said at the time that they had not been able to determine who was responsible for deploying the gas.
The more conclusive findings announced Thursday were aided by evidence sent to the United States by France, which, along with Britain, has announced it had determined that Assad's government had used chemical weapons.
Obama has said repeatedly that the use of chemical weapons would cross a "red line" and constitute a "game changer" for U.S. policy on Syria, which until now has focused entirely on providing the opposition with nonlethal assistance and humanitarian aid.
The White House said it had notified Congress, the United Nations and key international allies about the new U.S. chemical weapons determination. Obama will discuss the assessments, along with broader problems in Syria, next week during the G-8 summit in Northern Ireland.
Among those in attendance will be Russian President Vladimir Putin, one of Assad's most powerful backers. Obama and Putin will hold a one-on-one meeting on the sidelines of the summit, where the U.S. leader is expected to press his Russian counterpart to drop his political and military support for the Syrian government.
"We believe that Russia and all members of the international community should be concerned about the use of chemical weapons," Rhodes said.
Britain's U.N. Ambassador Mark Lyall Grant said his country was "not surprised by the determination made by the U.S. government," given its own assessments, and was in consultation with the Americans about next steps.
The U.S. has so far provided the Syrian rebel army with rations and medical supplies. In April, Kerry announced that the administration had agreed in principle to expand its military support to the opposition to include defensive items like night vision goggles, body armor and armored vehicles.
The Syrian fighters have been clamoring for bolder Western intervention, particularly given the estimated 5,000 Hezbollah guerrillas propping up Assad's forces. Assad's stunning military success last week at Qusair, near the Lebanese border, and preparations for offensives against Homs and Aleppo have made the matter more urgent.
While McCain has pressed for a greater role for the U.S. military, other lawmakers have expressed reservations about American involvement in another conflict and fears that weapons sent to the rebels could fall into the hands of al-Qaida-linked groups.
Sen. Chris Murphy, D-Conn., a member of the Senate Foreign Relations Committee, condemned the Assad regime but expressed serious concerns about the United States being pulled into a proxy war.
"There are many actions that the United States can take to increase our humanitarian assistance to refugee populations and opposition groups short of injecting more weapons into the conflict," Murphy said. "I urge the president to exercise restraint and to consult closely with Congress before undertaking any course of action to commit American military resources to Syrian opposition forces."
House Speaker John Boehner, R-Ohio, also urged the White House to consult with Congress.
"It is long past time to bring the Assad regime's bloodshed in Syria to an end," he said through a spokesman, Brendan Buck. "As President Obama examines his options, it is our hope he will properly consult with Congress before taking any action."
Text: US statement on Syria-chemical weapons
Text of a statement by Deputy National Security Adviser for Strategic Communications Ben Rhodes on the use of chemical weapons in Syria:
At the president's direction, the United States government has been closely monitoring the potential use of chemical weapons within Syria. Following the assessment made by our intelligence community in April, the President directed the intelligence community to seek credible and corroborated information to build on that assessment and establish the facts with some degree of certainty. Today, we are providing an updated version of our assessment to Congress and to the public.
The Syrian government's refusal to grant access to the United Nations to investigate any and all credible allegations of chemical weapons use has prevented a comprehensive investigation as called for by the international community. The Assad regime could prove that its request for an investigation was not just a diversionary tactic by granting the U.N. fact-finding mission immediate and unfettered access to conduct on-site investigations to help reveal the truth about chemical weapons use in Syria. While pushing for a U.N. investigation, the United States has also been working urgently with our partners and allies as well as individuals inside Syria, including the Syrian opposition, to procure, share, and evaluate information associated with reports of chemical weapons use so that we can establish the facts and determine what took place.
Following a deliberative review, our intelligence community assesses that the Assad regime has used chemical weapons, including the nerve agent sarin, on a small scale against the opposition multiple times in the last year. Our intelligence community has high confidence in that assessment given multiple, independent streams of information. The intelligence community estimates that 100 to 150 people have died from detected chemical weapons attacks in Syria to date; however, casualty data is likely incomplete. While the lethality of these attacks make up only a small portion of the catastrophic loss of life in Syria, which now stands at more than 90,000 deaths, the use of chemical weapons violates international norms and crosses clear red lines that have existed within the international community for decades. We believe that the Assad regime maintains control of these weapons. We have no reliable, corroborated reporting to indicate that the opposition in Syria has acquired or used chemical weapons.
The body of information used to make this intelligence assessment includes reporting regarding Syrian officials planning and executing regime chemical weapons attacks; reporting that includes descriptions of the time, location, and means of attack; and descriptions of physiological symptoms that are consistent with exposure to a chemical weapons agent. Some open source reports from social media outlets from Syrian opposition groups and other media sources are consistent with the information we have obtained regarding chemical weapons use and exposure. The assessment is further supported by laboratory analysis of physiological samples obtained from a number of individuals, which revealed exposure to sarin. Each positive result indicates that an individual was exposed to sarin, but it does not tell us how or where the individuals were exposed or who was responsible for the dissemination.
We are working with allies to present a credible, evidentiary case to share with the international community and the public. Since the creation of the U.N. fact-finding mission, we have provided two briefings to Dr. Åke Sellström, the head of the mission. We will also be providing a letter to UN Secretary General Ban, calling the U.N.'s attention to our updated intelligence assessment and specific incidents of alleged chemical weapons use. We request that the U.N. mission include these incidents in its ongoing investigation and report, as appropriate, on its findings. We will present additional information and continue to update Dr. Sellström as new developments emerge.
The president has been clear that the use of chemical weapons - or the transfer of chemical weapons to terrorist groups - is a red line for the United States, as there has long been an established norm within the international community against the use of chemical weapons. Our intelligence community now has a high confidence assessment that chemical weapons have been used on a small scale by the Assad regime in Syria. The President has said that the use of chemical weapons would change his calculus, and it has. Our decision making has already been guided by the April intelligence assessment and by the regime's escalation of horrific violence against its citizens. Following on the credible evidence that the regime has used chemical weapons against the Syrian people, the president has augmented the provision of non-lethal assistance to the civilian opposition, and also authorized the expansion of our assistance to the Supreme Military Council (SMC), and we will be consulting with Congress on these matters in the coming weeks. This effort is aimed at strengthening the effectiveness of the SMC, and helping to coordinate the provision of assistance by the United States and other partners and allies. Put simply, the Assad regime should know that its actions have led us to increase the scope and scale of assistance that we provide to the opposition, including direct support to the SMC. These efforts will increase going forward.
The United States and the international community have a number of other legal, financial, diplomatic, and military responses available. We are prepared for all contingencies, and we will make decisions on our own timeline. Any future action we take will be consistent with our national interest, and must advance our objectives, which include achieving a negotiated political settlement to establish an authority that can provide basic stability and administer state institutions; protecting the rights of all Syrians; securing unconventional and advanced conventional weapons; and countering terrorist activity.