A war of words erupted Sunday over Syria as Washington said it is ready to take action over chemical weapons attacks and Tehran warned US intervention would carry "harsh consequences".
Pressure mounted on Damascus to allow a UN probe of chemical attacks, with French President Francois Hollande saying evidence indicated the regime in war-ravaged Syria was to blame and Israel demanding action against its neighbour.
The Al-Nusra Front, a fierce Al-Qaeda-linked group fighting the regime of Bashar al-Assad, vowed revenge against villages of the Syrian president's minority Alawite community.
Doctors Without Borders said 355 people died this week of "neurotoxic" symptoms, after Syria's opposition claimed regime forces unleashed chemicals east and southwest of Damascus on Wednesday causing more than 1,300 deaths.
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.
US President Barack Obama 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.
His comments come a day after Obama held a rare meeting with his top aides and discussed Syria by phone with British Prime Minister David Cameron.
Cameron's office said the two leaders agreed the use of chemical weapons would "merit a serious response" -- echoing French calls.
On Sunday Defence Secretary Chuck Hagel said the US military was "prepared to exercise whatever option" against Syria but intelligence was still being evaluated.
Syria denies it used chemical weapons and has reportedly said it will work with UN inspectors who have been on the ground for a week to probe three other suspect sites.
"The Syrian government will cooperate with the United Nations mission now in Syria to create the conditions for a visit to zones where terrorist groups have carried out attacks with chemical weapons," Iranian Foreign Minister Mohammad Javad Zarif said.
He said Muallem gave him the pledge during a phone conversation but there was no independent confirmation.
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US Secretary of State John Kerry told Muallem this week that if Syria has "nothing to hide" it should let the UN experts inspect the site before evidence is destroyed.
On a visit to Malaysia, Hagel said the US defence department had prepared "options for all contingencies" at Obama's request.
"Again, we are prepared to exercise whatever option, if he decides to employ one of those options," he added.
Obama and Cameron "are both gravely concerned by...increasing signs that this was a significant chemical weapons attack carried out by the Syrian regime against its own people," the British leader's office said.
Several world leaders have said they believe Syria's regime is behind the chemical attacks and are demanding it gives the inspectors unfettered access.
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".
UN High Representative for Disarmament Affairs Angela Kane was tasked by Secretary General Ban Ki-moon with establishing the terms of an inquiry.
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 that "scientific" proof is still lacking.
"Syrian doctors we work with have no scientific proof. They must take hair samples, for example, and send them to a specialist laboratory," to carry out conclusive tests, he said.
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.
As Syria and its opponents traded accusations on who used chemicals, the radical Al-Nusra Front vowed punitive action.
"The Alawite villages will pay the price for each chemical rocket that struck our people in Damascus," the group's leader Abu Mohammed al-Jawlani said in an audio message posted online.
More than 100,000 people have been killed in Syria since an uprising against Assad's rule flared in March 2011, the UN says.