UN experts are to start investigating a suspected Syrian chemical attack Monday as a sceptical Washington weighing military action and coordinating with allies said Syria's acceptance of the probe came too late.
In an escalation of a showdown over the alleged chemical weapons attack near Damascus last week, the United States and its Western allies pointed the finger of blame at President Bashar al-Assad's regime.
"There is very little doubt at this point that a chemical weapon was used by the Syrian regime against civilians in this incident," based on the reported number of victims and their symptoms, as well as US and other foreign intelligence, one official in Washington told AFP. Read: Snipers shoot at UN chemical weapons team in Syria
French President Francois Hollande told his US counterpart Barack Obama that "everything was consistent" with the conclusion that Damascus was behind the attack.
"The two presidents agreed to stay in close contact to arrive at a joint response to this unprecedented aggression," the French leader's office said.
And British Foreign Secretary William Hague warned any evidence of a chemical attack may have been destroyed. "The fact is that much of the evidence could have been destroyed by that artillery bombardment," he said.
However, French Prime Minister Jean-Marc Ayrault implied that a punitive strike on Syria was not imminent, in an interview with France 2 television.
"Once this (UN) investigation has ended, we await a firm decision, a clear decision, from the international community. The (UN) Security Council will meet," he said.
A White House official also shot down a report in Britain's Telegraph newspaper claiming that London and Washington planned to join forces and launch military action against Syria "within days". Read: US army ready to act over Syria chemical claims
"The president has not made a decision to undertake military action," the official said.
Syria's opposition says more than 1,300 people died when regime forces unleashed chemical weapons against rebel-held towns east and southwest of Damascus Wednesday, while Doctors Without Borders said 355 people had died of "neurotoxic" symptoms.
Damascus has strongly denied it carried out such an attack, instead blaming the rebels.
With the drums of a wider war beating, Syria's ally Moscow bluntly warned the West that military action against Assad's regime would be a "tragic mistake". Read: Russia warns US against 'extremely dangerous' Syria attack
Syria's foreign ministry said that visiting UN disarmament envoy Angela Kane on Sunday struck the accord with the government for a probe.
The United Nations said the investigation would begin as early as Monday, stressing that the rebels and government were responsible for the safety of the UN inspectors on the ground and that a local ceasefire had been agreed.
US officials said Obama, who held crisis talks Saturday with top security aides, would make an "informed decision" about how to respond to an "indiscriminate" chemical weapons attack.
Washington has noted that Syria offered to let UN inspectors view the site of the alleged attack, but suggested it was too little, too late, said one official, speaking on condition of anonymity. Read: China calls for 'cautious' approach on Syria chemical weapons
"At this juncture, the belated decision by the regime to grant access to the UN team is too late to be credible, including because the evidence available has been significantly corrupted as a result of the regime's persistent shelling and other intentional actions over the last five days," the official said.
If confirmed, the attack would mark the deadliest use of chemical agents since Iraqi dictator Saddam Hussein gassed Iranian troops and Kurdish rebels in the 1980s.
US defence secretary Chuck Hagel said earlier Sunday the US military was "prepared to exercise whatever option" against Syria but intelligence was still being evaluated.
On a visit to Malaysia, Hagel said the US defence department had prepared "options for all contingencies" at Obama's request.
Obama had said a year ago that the use of chemical weapons by Assad's forces was a "red line" that could trigger Western intervention.
On Sunday, a strident warning came from Washington's archfoe Iran. Read: Turkey would join coalition against Syria, says foreign minister
"If the United States crosses this red line, there will be harsh consequences for the White House," armed forces deputy chief of staff Massoud Jazayeri said, without elaborating.
The Arab League is to meet on Tuesday to discuss the alleged use of chemicals, the bloc's deputy chief Ahmed Ben Helli said.
In Israel, President Shimon Peres called for international efforts to "take out" chemical weapons in Syria as Prime Minister Benjamin Netanyahu said Israel would pull the "trigger" if needed to protect its people.
More than 100,000 people have been killed in Syria since an uprising against Assad's rule flared in March 2011, the UN says.
In the latest eruption of violence, the governor of Hama province in central Syria was killed in a car bombing on Sunday, state television reported, in an attack it blamed on rebels.