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.
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.
U.S. senators John McCain and Lindsey Graham, who have criticized the Obama administration's hands-off approach to Syria, said in a joint statement that in light of the latest suspected chemical attack, "now is the time for decisive actions."
"The United States must rally our friends and allies to take limited military actions in Syria that can change the balance of power on the ground and create conditions for a negotiated end to the conflict and an end to Assad's rule," the statement said.
In Paris, French President Francois Hollande said a "body of evidence" suggests that chemical weapons were used during last week's attacks, and "everything" leads France to believe that the Assad regime was behind it.
Conveying new urgency about the situation, Hollande's office said he spoke about Syria by telephone Sunday with Obama, as well as prime ministers David Cameron of Britain and Kevin Rudd of Australia. The White House said in a statement the two leaders discussed "possible responses by the international community and agreed to continue to consult closely."
Just over a year ago, Obama called the use of chemical arms in Syria a "red line" that would carry "enormous consequences." Since then, U.S. intelligence believes that such weapons have been used on a small scale several times, but that has precipitated no major shift in American policy or substantial action against the Assad regime.
So far, the U.S. has largely limited its support for the rebels to non-lethal supplies. In June, Washington said it would begin sending weapons to the rebels, although there's no indication that has happened yet.
Russia, a close ally of the Assad regime, welcomed Syria's decision to allow a U.N. probe, and said the U.S. should await the findings and realize that a unilateral use of force would be a mistake.
Foreign Ministry spokesman Alexander Lukashevich said Washington and European partners shouldn't take a "gamble" that could have "catastrophic consequences" for Syria and the region as a whole.
In Damascus, Syrian Information Minister Omran al-Zoubi said the United States was using allegations of chemical attacks as an "excuse" to intervene in Syria, accusing Washington and Europe of turning a blind eye while Saudi Arabia and Turkey - both backers of the anti-Assad rebellion - provide chemical weapons to foreign jihadi fighters in Syria.
"The materials are coming from Saudi Arabia and Turkey," al-Zoubi said in an interview with The Associated Press in the Syrian capital, adding that foreign fighters were carrying out chemical attacks to implicate the Syrian government in hopes of prompting international military intervention.
"Instead of the Americans searching for the source of these chemical weapons in Saudi Arabia and Turkey, they wait for them to be used to give a pretext to intervene in Syria," he said.
Iran, a close ally of the Assad regime, warned against a possible U.S. military move against Syria. The semi-official Fars news agency quoted Gen. Masoud Jazayeri as warning that "trespassing over the red line in Syria will have severe consequences for the White House." He did not elaborate.
Doctors Without Borders said that three hospitals it supports in the area of the attack reported receiving roughly 3,600 patients with "neurotoxic symptoms" over a three-hour period Wednesday. Of those, 355 died, the Paris-based group said.
That roughly coincided with the death toll given by the Britain-based Syrian Observatory for Human Rights, which tallied 322 killed. Casualty figures have varied widely over the alleged attack, from just over 300 to around 1,300 killed.
Rebels on the ground outraged by the images coming out of the Damascus suburbs have also threatened to take forceful action.
On Sunday, the head of an al-Qaida-linked Syrian rebel group vowed to target villages inhabited by members of Assad's minority Alawite sect to avenge the purported chemical weapons attack.
In an audio recording posted on a website frequently used by Islamic extremists, Jabhat al-Nusra leader, Abu Mohammed al-Golani, said: "Revenge for the blood of your children is a debt to be paid back ... 1,000 rockets will be fired at them in revenge for the massacre of Ghouta."
The authenticity of the claim could not be immediately verified.
Lucas reported from Beirut. Associated Press writers Jamey Keaten in Paris, Jamal Halaby in Amman, Jordan, and Kimberly Dozier and Lolita C. Baldor in Washington contributed to this report.