DAMASCUS, Syria (AP) -- Syria agreed Sunday to a U.N. investigation into last week's alleged chemical weapons attack outside Damascus -- a deal a senior White House official dismissed as "too late to be credible," saying the United States has "very little doubt" President Bashar Assad's forces used such weapons.
The hardening of the U.S. position came as calls for military action grow. In a sign the U.S. may be a step closer to an armed response, naval forces have already been dispatched toward Syria's coastal waters, although President Barack Obama has cautioned against a hasty decision.
With France, Britain, Israel and some U.S. congressmen urging swift military action against Assad's regime if the use of chemical agents is confirmed, the U.N. team's conclusions could have a dramatic impact on the trajectory of the country's civil war.
The agreement struck in Damascus calls for U.N. experts already in the country to begin an investigation Monday into the suspected chemical attack on rebel-held areas in the capital's eastern suburbs.
Anti-government activists and Doctors Without Borders say that more than 300 people were killed in an artillery barrage by regime forces Wednesday that included the use of toxic gas. The government calls the allegations "absolutely baseless."
The suburbs hit in the suspected chemical strike, collectively known as eastern Ghouta, are under the control of rebel fighters, and regime artillery and warplanes have pounded the area for days. The U.N. inspectors will have to traverse through both government-held and opposition-controlled turf to conduct their probe. Rebels have said they will help facilitate the visit.
Under Sunday's agreement with the U.N., the Syrian government "affirmed that it will provide the necessary cooperation, including the observance of the cessation of hostilities at the locations related to the incident," U.N. spokesman Martin Nesirky said in a statement.
In Washington, a senior administration official said the U.S. has "very little doubt" that regime forces used chemical weapons in Wednesday's attack, an assessment that was "based on the reported number of victims, reported symptoms of those who were killed or injured" as well as witness accounts and facts gathered by the U.S intelligence community.
The official, who insisted on anonymity because of lack of authorization to speak publicly about the developments, was dismissive of the Syrian government's agreement to grant access to the U.N. team, saying it was "too late to be credible."
The regime's continuing shelling of the site would have "significantly corrupted" any available evidence of chemical weapons, the official said.
The U.N. team was in Syria to look into three earlier suspected chemical attacks, with a mandate to determine whether such weapons were used, not who was responsible for unleashing them. There was no indication that the mission's brief had been expanded to assess who was behind Wednesday's attack.
Even as the pressure mounts for a strong international response, there is no guarantee that foreign powers will take action if the U.N. confirms chemical agents were used. But the scale of the attack makes this instance far harder to ignore than previous suspected cases.
A senior State Department official, not authorized to comment publicly by name, said U.S. Secretary of State John Kerry spoke with the top diplomats of Britain, France, Canada and Russia as well as U.N. Secretary Ban Ki-Moon.
The official said Kerry stressed that if the Syrian regime wanted to prove to the world that it had not used chemical weapons in this incident, it would have stopped shelling the area and granted immediate access five days ago.
U.S. Defense Secretary Chuck Hagel offered no hints Sunday about likely U.S. responses, telling reporters traveling with him in Malaysia that the Obama administration was still assessing intelligence about Wednesday's attack.
"When we have more information, that answer will become clear," he said when a reporter asked whether it was a matter of when, not if, the U.S. will take military action against Syria.
"There are risks and consequences for any option that would be used or not used -- for action or inaction," he told reporters. "You have to come to the central point of what would be the objective if you are to pursue an action or not pursue an action. So all those assessments are being made."
The U.S. has about a dozen F-16 jets, a Patriot missile battery and as many as 1,000 American troops in Jordan, which all could also be used in any military action. U.S. administration and defense officials in recent days have said the most likely military move would be the launch of Tomahawk missiles off ships in the Mediterranean.