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 and 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.
"They have a history of lies — Iraq," he said.
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 — and the White House dismissed as "fanciful" the notion that anyone other than Assad could be to blame.
"Suggestions that there's any doubt about who's responsible for this are as preposterous as a suggestion that the attack did not occur," spokesman Jay Carney said.
A U.S. official said some of the evidence includes signals intelligence — 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 because they were not authorized to publicly discuss the internal deliberations.
The United Nations said its team of chemical weapons experts in Syria had delayed a second trip to investigate the alleged attack by one day for security reasons. On Monday, the team came under sniper fire.
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.