The use of chemical weapons by either side is a nightmare scenario. Along with its warnings about Assad, the West is just as concerned that rebel forces, including some linked to al-Qaida, could get their hands on Syria's chemical weapons supplies.
Despite the importance, any clear confirmation of the nature of the attack that took place Tuesday in the northern village of Khan al-Assal, killing at least 31 people, is unlikely. Syria's government seals off areas it controls to journalists and outside observers.
The two sides blamed each other for a chemical attack without offering clear proof or documentation, as has frequently been the case in the Syrian civil war.
If confirmed, it would be the first time a chemical weapon has been used in Syria's war that has already killed an estimated 70,000 people.
Syria's U.N. Ambassador Bashar Jaafari told reporters at the United Nations Wednesday that he had asked U.N. Secretary-General Ban Ki-moon to form "a specialized, independent and neutral technical mission to investigate the use by the terrorist groups operating in Syria of chemical weapons" in Khan al-Assal.
Jaafari called the attack "very serious and alarming and unacceptable and unethical."
U.N. spokesman Martin Nesirky said he would have something to say "once we receive any formal request, which we have so far not received." He said the secretary-general remains convinced that the use of chemical weapons by any party under any circumstances would constitute "an outrageous crime."
Syria's main opposition group also demanded an international investigation.
"All evidence now indicates that the Assad regime is using these weapons against its own people," the main Western-backed opposition group, the Syrian National Coalition, said.
"The Coalition demands a full international investigation, and asks for a delegation to be sent to inquire and visit the site," the group said in a statement.
Obama has declared the use, deployment or transfer of the weapons to be his "red line" for possible military intervention in the Arab country.
"When you start seeing weapons that can cause potential devastation and mass casualties and you let that genie out of the bottle, then you are looking at potentially even more horrific scenes than we've already seen in Syria," Obama said in Jerusalem, "and the international community has to act on that information."
Russia and Iran, Assad's main allies, backed his regime's charges.
Iranian Foreign Ministry spokesman Ramin Mehmanparast blamed "armed opposition groups," calling use of chemical weapons "an inhuman act."
"Undoubtedly, the responsibilities of a repetition of such crimes would fall on those committing it and the countries that support them," he was quoted by state TV as saying, apparently referring to Gulf states such as Qatar and Saudi Arabia
"There's definitely a propaganda war between the regime and opposition," said Ayham Kamel, a Middle East analyst at the Eurasia Group in London.
"Because we cannot verify either claims, we are going to be stuck in the same cycle of accusations, unless some international mission is actually sent there to verify what happened," he said.
Jordan's king warned Wednesday that an extremist Islamic state could form on his border.
King Abdullah II told The Associated Press in an interview that in his view, Assad was beyond rehabilitation, and it was only a matter of time before his authoritarian regime collapses.
"The most worrying factors in the Syrian conflict are the issues of chemical weapons, the steady flow or sudden surge in refugees and a jihadist state emerging out of the conflict," the king said.
The opposition's disunity was on display again Wednesday.
About a dozen members of the Syrian National Coalition suspended their membership a day after it elected the first rebel prime minister.
Among them were senior members including Suheir Atassi, Kamal Labwani, and spokesman Waleed al-Bunni.
Atassi said explained why she suspended her membership. "I refuse to be a follower and I refuse to be simply a woman who decorates their gatherings and conferences while they make all the decisions," she wrote on her Facebook page.
Coalition members have complained of the dominance of the fundamentalist Muslim Brotherhood in the SNC, and Hitto was one of the top Brotherhood candidates.
In fighting Wednesday, activists reported intense clashes in the Quneitra region on the cease-fire line between Syria and Israel in the Golan Heights.
The Britain-based Syrian Observatory for Human Rights said rebels seized control of parts of villages a few miles from the cease-fire line after fierce fighting with regime forces.
It said seven people, including three children, were killed by government shelling villages.
Activists on Facebook pages affiliated with rebels in Quneitra announced the start of the operation to "break the siege on Quneitra and Damascus' western suburbs."
The fall of Quneitra in rebel hands would be significant because it is close to the Israeli frontier.
Israel has said its policy is not to get involved in the Syrian civil war, but it has retaliated to sporadic Syrian fire that has spilled over into Israeli communities in the Golan Heights.
Also Wednesday, Assad made a rare public appearance, visiting a fine arts school in Damascus and meeting the parents of students who were killed in the civil war, state TV reported.
Photos run by the Syrian state media showed Assad shaking hands and listening closely to people who were said to be parents of war victims. It was his first appearance outside his palace since January, when he delivered a speech.
"All of Syria is wounded, and there is no one who hasn't lost a relative, a brother, a father or a mother," Assad said, according to the official news agency SANA.
"However, all that is happening cannot make us weak, and the battle is a battle of will and steadfastness," he added.