The United States said Sunday there was "very little doubt" Syrian forces had used chemical weapons on civilians, and dismissed an offer by Damascus for a UN team to view the attack site.
The comments marked a significant escalation of a showdown over the horrific attack outside the Syrian capital that killed up to 1,300 people last week, and came as Washington appeared to be positioning for possible military action.
Officials said President Barack Obama, who held crisis talks Saturday with top aides, would make an "informed decision" about how to respond to an "indiscriminate" chemical weapons attack.
An official told AFP that based on the reported number of victims and their symptoms, and US and foreign intelligence, "there is very little doubt at this point that a chemical weapon was used by the Syrian regime against civilians in this incident."
The official, speaking on condition of anonymity, said that Washington had noted that Syria had offered to let UN inspectors view the site of the alleged attack on Monday, but said it was too little, too late, and that the evidence available at the site "has been significantly corrupted" due to the delay.
A US diplomatic offensive led by Secretary of State John Kerry, comments coming from the White House and signs the Pentagon is positioning ships closer to Syria fueled an impression that Obama may be getting ready to jettison his antipathy to new Middle Eastern entanglements and to order limited military action.
Kerry has spent days on the phone with Washington's foreign partners.
On Sunday Kerry spoke with UN Secretary General Ban Ki-moon, as well as his British, French, Canadian and Russian counterparts, a senior State Department official said, speaking on condition of anonymity.
In all of the calls 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," the official said.
Kerry "made clear that ... there is very little doubt that a chemical weapon was used by the Syrian regime against civilians in this incident," based on intelligence reports, and information from US international partners.
Kerry also reiterated that Obama "is studying the facts and will be making an informed decision about the responsible way forward," the official said.
The United Nations meanwhile said that its team would start a probe on Monday into what happened in the area, which is under rebel control.
President Bashar al-Assad's government said the visit would prove that claims by the opposition that chemical weapons were used against defenseless civilians, including children, were "lies."
The opposition says 1,300 people were killed. Doctors Without Borders has said 355 people died in hospital alone from "neurotoxic" symptoms.
French President Francois Hollande told Obama on Sunday in a phone call that "everything was consistent" with the conclusion that Damascus was behind last Wednesday's suspected chemical attack.
"The two presidents agreed to stay in close contact to arrive at a joint response to this unprecedented aggression," the French government said in a statement.
Russia however reacted to US maneuvering by warning Washington against making a "tragic mistake."
Polls show Americans are wary of getting into another war in the Middle East.
But Obama's credibility is on the line after he said last year that the use of large-scale chemical weapons in Syria would cross a US "red line."
Washington may also want to send a signal that weapons of mass destruction, like chemical arms, cannot be used in Syria, or elsewhere, with impunity.
Republican senators John McCain and Lindsey Graham, who have criticized Obama's failure to use military force to protect civilians in a war that has killed 100,000 people, upped pressure on the president on Sunday.
"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 senators said in a statement.
"Using stand-off weapons, without boots on the ground, and at minimal risk to our men and women in uniform, we can significantly degrade Assad's air power and ballistic missile capabilities and help to establish and defend safe areas on the ground."
Experts believe that the most likely US action would see sea-launched cruise missiles target Syrian military installations and artillery batteries deemed complicit in the chemical weapons attack.
Weapons fired from US planes outside the country could also be used, to minimize the risk to US or allied pilots from Syrian air defenses.
Washington would likely seek to act with a broad coalition of European and Gulf allies as Russia is seen sure to veto any attempt to mandate action against its Middle Eastern ally in the UN Security Council.
Obama called British Prime Minister David Cameron on Saturday to discuss options, after a meeting of all his top diplomatic, military and intelligence chiefs at the White House.
US Defense Secretary Chuck Hagel said on a tour of Asia that the military would be ready to act if called on by Obama.
"President Obama has asked the Defense Department to prepare options for all contingencies. We have done that," Hagel said.