UN experts are to start investigating an alleged Syrian chemical weapons site today after receiving the go-ahead from Damascus, as a sceptical Washington said Syria's acceptance had come too late.
In an escalation of a showdown over an alleged chemical weapons attack near Damascus last week, the United States pointed the finger of blame at President Bashar al-Assad's regime as it weighed military action.
"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.
Moscow bluntly warned the West that military action against the Syrian regime would be a "tragic mistake''.
Syria's foreign ministry said that visiting UN disarmament envoy Angela Kane, tasked by Secretary General Ban Ki-moon to establish the terms of an inquiry, struck an accord Sunday with the Syrian government for a probe.
The United Nations said in a statement the investigation would begin as early as Monday.
Ban "has instructed the mission ... currently in Damascus, to focus its attention on ascertaining the facts of the 21 August incident as its highest priority,'' the UN said in a statement.
"The mission is preparing to conduct on-site fact-finding activities, starting tomorrow, Monday, 26 August.''
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 on August 21, while Doctors Without Borders said 355 people had died of "neurotoxic'' symptoms.
Damascus has strongly denied it carried out an attack using chemical arms, instead blaming the rebels.
US 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.
Washington had noted that Syria had offered to let UN inspectors view the site of the alleged attack, but suggested it was too little, too late, one official, speaking on condition of anonymity.
"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,'' he said.
US Defence Secretary Chuck Hagel said earlier 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.
French President Francois Hollande said evidence indicated the regime was to blame for the chemical attacks, while Israel demanded action against its Arab neighbour.
Hollande said there was "a body of evidence indicating that the August 21 attack was chemical in nature, and that everything led to the belief that the Syrian regime was responsible for this unspeakable act''.
If confirmed, it would be the deadliest use of chemical agents since late Iraqi dictator Saddam Hussein gassed Iranian troops and Kurdish rebels in the 1980s.
Al-Nusra Front, a fierce Al-Qaeda-linked group fighting the regime, vowed revenge against villages of Assad's minority Alawite community.
On Saturday, Obama held a rare meeting with his top aides and discussed Syria by phone with British Prime Minister David Cameron.
Cameron's office said they two leaders agreed the use of chemical weapons would "merit a serious response'' - echoing French calls.
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.
"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 chemical weapons in Syria, the bloc's deputy chief Ahmed Ben Helli said.
Doctors Without Borders (MSF) has said about 3,600 patients displaying "neurotoxic symptoms'' had flooded into three Syrian hospitals on the day of the alleged attacks, and 355 of them died.
"Medical staff working in these facilities provided detailed information to MSF doctors regarding large numbers of patients arriving with symptoms including convulsions, excess saliva, pinpoint pupils, blurred vision and respiratory distress,'' said MSF operations director Bart Janssens.
MSF president Mego Terzian told AFP however that "scientific'' proof is still lacking.
In Israel, President Shimon Peres called for international efforts to "take out'' chemical weapons in Syria as Prime Minister Benjamin Netanyahu said Israel will 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.