Three hospitals near Damascus reported 355 deaths after an alleged chemical attack last Wednesday out of around 3,600 admissions with nerve gas-type symptoms, the medical charity Medecins Sans Frontieres (MSF) has said .

The Syrian opposition accused government forces of gassing hundreds of people on Wednesday by firing rockets that released deadly fumes into rebel-held Damascus suburbs, killing men, women and children as they slept. Opposition estimates for the death toll have ranged from 500 to well over double that number, but, with UN observers unable to visit the site, there has been no independent verification.

MSF has no staff of its own in the Damascus region, but has been supporting hospitals and medical networks there since 2012. “The reported symptoms of the patients, in addition to the epidemiological pattern of the events - characterised by the massive influx of patients in a short period of time, the origin of the patients, and the contamination of medical and first aid workers - strongly indicate mass exposure to a neurotoxic agent,” MSF director of operations Bart Janssens said in a statement.

“This would constitute a violation of international humanitarian law, which absolutely prohibits the use of chemical and biological weapons.” Mr Janssens said MSF could not confirm the cause of the symptoms or say who was responsible for the attack, but that it had sent 7,000 vials of atropine - an antidote against nerve agents.

Meanwhile military and national security advisers huddled with US president Barack Obama at the White House on Saturday to consider options for responding to the alleged use of chemical weapons by the Syrian government this week.

Mr Obama has been reluctant to intervene in Syria’s 2-1/2-year civil war, which he has described as a “sectarian complex problem.” But a year ago he said chemical weapons would be a “red line” for the United States and he is now under pressure to take action.

Vehicles pulled up in front of the White House’s West Wing on Saturday morning, bringing officials to the meeting. US secretary of state John Kerry and US secretary of Defence Chuck Hagel, who is in Asia, both participated remotely.

White House officials were cautious in describing the content of the discussions. “We have a range of options available, and we are going to act very deliberately so that we’re making decisions consistent with our national interest as well as our assessment of what can advance our objectives in Syria,” said a White House official.

Mr Obama said in a CNN interview broadcast on Friday that the United States was still gathering information about the apparent chemical weapons attack that killed as many as 1,000 civilians in a Damascus suburb.

He noted, however, that chemicals weapon use on a large scale would start “getting to some core national interests that the United States has, both in terms of us making sure that weapons of mass destruction are not proliferating, as well as needing to protect our allies, our bases in the region.”

The US Navy repositioned a ship armed with cruise missiles in the Mediterranean yesterday. Mr Hagel said Mr Obama had asked the Pentagon for options on Syria.

American and European security sources said yesterday that US and allied intelligence agencies had made a preliminary assessment that chemical weapons were used by Syrian forces in the attack near Damascus this week.

Separately the UN High Representative for Disarmament Affairs Angela Kane arrived in Syria today to push for access to a suspected chemical weapons attack site for UN inspectors, who are already in Syria to investigate previous accusations.

Ms Kane arrived at the Four Seasons hotel in Damascus at midday local time , a Reuters reporter at the scene said.

The US and the UK have pressed Assad’s government to allow United Nations inspectors to enter the site of the alleged attack to gather evidence.

French foreign minister Laurent Fabius called on the world to respond “with force” to any use of chemical weapons.

Internet video and photos showed dead Syrians without visible wounds after the attack, and reports from local doctors were consistent with nerve gas or lethal exposure to pesticides.

US intervention in Syria must be part of a broad international effort, US Defense Secretary Chuck Hagel said, echoing Mr Obama’s comments in an interview broadcast on CNN earlier yesterday that the US would need a UN mandate to act in Syria.

Mr Hagel declined to specify how soon the US would decide if military action is warranted against Assad’s regime.

“The international community is moving swiftly in getting facts on what did happen and getting the intelligence right and all the other factors that go into a decision will be made swiftly and should be made swiftly,” he said.

Agencies