British Prime Minister David Cameron recalled Parliament to hold an emergency vote Thursday on his country's response. It is unlikely that any international military action would begin before then.
U.S. Defense Secretary Chuck Hagel said U.S. military forces stand ready to strike Syria at once if President Barack Obama gives the order, and French President Francois Hollande said France was "ready to punish those who took the heinous decision to gas innocents."
Obama is weighing a response focused narrowly on punishing Assad for violating international agreements that ban the use of chemical weapons. Officials said the goal was not to drive Assad from power or impact the broader trajectory of Syria's bloody civil war, now in its third year.
Russian Foreign Minister Sergey Lavrov said Monday the West should be under no illusion that bombing Syrian military targets would help end the violence in Syria, an ally of Moscow. He pointed to the volatile situations in Iraq and Libya that he said resulted from foreign military intervention.
Syrian Foreign Minister Walid al-Moallem said his country would use "all means available" to defend itself.
"We have the means to defend ourselves, and we will surprise everyone," he said.
At a news conference in Damascus, al-Moallem challenged Washington to present proof to back up its accusations. He also likened the allegations to false American charges in 2003 that Iraq possessed weapons of mass destruction before the U.S.-led invasion of that country.
Vice President Joe Biden said there was no question that Assad was responsible for the attack the highest-ranking U.S. official to say so.
A U.S. official said some of the evidence includes information gathered from intercepted communications. The U.S. assessment is also based on the number of reported victims, the symptoms of those injured or killed, and witness accounts. The officials insisted on anonymity.
The U.N. said its team of chemical weapons experts in Syria had delayed a second trip to investigate the alleged attack by a day for security reasons.
If Obama decides to order an attack against Syria, it would most likely involve sea-launched cruise missile attacks on Syrian military and communications targets. The U.S. Navy has four destroyers in the eastern Mediterranean Sea within range of targets inside Syria. The U.S. also has warplanes in the region.