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The White House says there is "very little doubt" that the Assad regime is responsible for the alleged chemical attack in Syria that is said to have taken place earlier this week.

"Based on the reported number of victims, reported symptoms of those who were killed or injured, witness accounts, and other facts," a senior administration official tells ABC News, "There is very little doubt at this point that a chemical weapon was used by the Syrian regime against civilians in this incident. We are continuing to assess the facts so the President can make an informed decision about how to respond to this indiscriminate use of chemical weapons."

White House officials also point out the attack was on rebel-held territory and apparently done using rockets that the rebels do not possess.

After reports that Syria's regime will allow UN inspectors to access the site of the attack, the senior administration official suggested the move may be too late, after "the regime's persistent shelling" has "significantly corrupted" evidence in the area.

"If the Syrian government had nothing to hide and wanted to prove to the world that it had not used chemical weapons in this incident, it would have ceased its attacks on the area and granted immediate access to the UN-five days ago. At this juncture, the belated decision by the regime to grant access to the UN team is too late to be credible," the official said.

The president has ordered his national security team to draw up possible strike options on Syria, but there is a divide in the White House on how forcefully to respond, although another official told ABC News if there is a strike, it must be "timely" - done soon enough to prevent another chemical attack.

Read: Red Line Crossed? Syria in Focus at White House Meeting

However, the White House does not want to act alone. U.S. officials are back channeling through the United Nations to see if Russia could be convinced to agree to a resolution.

If there is no UN authorization, the United States would lead any possible strike, but, a senior official told ABC News "we do not want to do anything on our own." U.S. allies must commit both "resources" and "political will" the official said.

On "This Week" Sunday, former Secretary-General of the Arab League Amr Moussa told ABC News' Martha Raddatz that "any possible use of chemical weapons should be condemned," but that the UN Security Council should consider the issue before any action is taken against Syria.

"If there is, or there are indications that chemical weapons were used, then the security council should consider the matter first before anything else," Moussa said. "This is a very serious situation. And let everybody express his view or their, whether it is the United States, or Russia or Europe."

"This is a very dangerous thing," Moussa added. "But things have to be verified in order to take a decision just like that, an air strike or invasion or whatever it is."

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