DAMASCUS (AFP) ― Pressure mounted Sunday on Damascus to allow a U.N. probe into claims of chemical weapons attacks, as Washington and London said there are “increasing signs” Syria‘s regime is to blame.

A senior United Nations envoy is in Damascus pressing for an investigation by U.N. inspectors who have been on the ground in war-ravaged Syria for a week to probe three other suspect sites.

The opposition accuses President Bashar al-Assad’s regime of killing more than 1,300 people Wednesday in chemical attacks east and southwest of Damascus.

Doctors Without Borders said 355 people died this week of “neurotoxic” symptoms.

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.

U.S. President Barack Obama said a year ago that the use of chemical weapons by Assad‘s forces was a “red line” that could bring about a more strident Western intervention in Syria’s civil war.

After a rare meeting with his top security aides on Saturday, Obama spoke by phone with British Prime Minister David Cameron about the alleged attacks, and a statement said their use would “merit a serious response”.

And on Sunday Defense Secretary Chuck Hagel said the U.S. military was “prepared to exercise whatever option” against Syria if ordered, but added that intelligence was still being gathered.

But the Iranian armed forces deputy chief of staff, Massoud Jazayeri, warned on Sunday that “if the United States crosses this red line, there will be harsh consequences for the White House,” according to Fars news agency.

The foreign minister of Iran, Syria‘s chief regional ally, said earlier Damascus would facilitate a U.N. visit to probe the allegations, although there was no immediate confirmation of that.

“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,” said Mohammad Javad Zarif.

His remarks came after a telephone conversation with Syrian counterpart Walid Muallem.

The Iranian minister also spoke with U.S. Secretary of State John Kerry, who told him that if Syria has “nothing to hide” it should let the U.N. experts inspect the site before evidence is destroyed.

Under mounting pressure to act, Obama has instructed commanders to chart “options” and directed intelligence chiefs to gather facts and met top aides and brass, the White House said.

“President Obama has asked the Defense Department to prepare options for all contingencies. We have done that,” Hagel said in Kuala Lumpur.

“Again, we are prepared to exercise whatever option, if he decides to employ one of those options,” he said, adding the U.S. was assessing intelligence.

Obama and Cameron discussed Syria on Saturday and “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 premier’s office said.

“The fact that President Assad has failed to cooperate with the U.N. suggests that the regime has something to hide,” it said.

“Significant use of chemical weapons would merit a serious response from the international community” ― echoing French calls for the use of “force.”

Australian Prime Minister Kevin Rudd on Sunday also spoke of indications that chemical weapons were used and said it was up to Syria‘s government to prove its hands were clean.

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.

But he insisted “MSF cannot confirm scientifically the cause of these symptoms nor establish responsibility for this attack.”

The Syrian Observatory for Human Rights said more than 300 people died from the effects of gas, including 82 women and 54 children.

U.N. High Representative for Disarmament Affairs Angela Kane was tasked by Secretary General Ban Ki-moon to establish the terms of an inquiry.

Reports of violence continued to pour from the battlefronts and state media charged that rebels used chemical weapons on army troops in Jobar, a northwestern district of Damascus.

State television said several soldiers suffered from asphyxiation.

The opposition National Coalition denied the claims, accusing the government of diversion tactics.

The United Nations says more than 100,000 people have been killed in Syria since an uprising against Assad’s rule flared in March 2011, while millions more have fled the country or been internally displaced.