Syrian state media says the government has reached an agreement to allow a UN team of experts to visit the site of last week's alleged chemical weapons attack as military action against Syria moved a step closer after Britain and the United States warned President Bashar al-Assad that there would be a “serious response” to the massacre. As fresh footage emerged of children killed in the strike against a Damascus suburb last Wednesday, David Cameron and Barack Obama agreed in a 40-minute phone call yesterday that the Syrian government was responsible for the atrocity and that all military options are on the table.

The two leaders agreed that "a significant use of chemical weapons would merit a serious response" and a "new stage" in the two-and-a-half-year civil war. In a sign of how the military build-up is escalating, Washington despatched a fourth naval warship to the Mediterranean capable of launching missile attacks on targets on the Syrian mainland. Mr Cameron and the US President are "looking for a response that makes clear our abhorrence about the use of chemical weapons", a British source said. "Both shared the view that there is little doubt that this was a significant use by the Assad regime of chemical weapons against his own people," a No 10 source said.

While Britain is against sending "boots on the ground", military action could range from enforcing no-fly zones to air strikes against the Syrian regime.

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Britain and the US are demanding that Assad now allows UN weapons inspectors, who are 12 miles away from the scene of Wednesday's attack, to examine the site. President Obama warned last year that a chemical weapons attack would be a "red line" for the US in the crisis. A No 10 source added: "As the days pass, they [Cameron and Obama] think it is increasingly unlikely that inspectors will get in."

The attack, believed to involve a nerve agent, killed as many as 1,000 civilians, including children, and wounded thousands of others, according to Syrian rebel sources.

The charity Médecins Sans Frontières said yesterday that hospitals it supports around Damascus have received about 3,600 patients over the past few days, showing "neurotoxic symptoms", of which 355 have already died. It is feared there could have been hundreds more deaths.

As outrage over the atrocity fuelled calls for action, President Obama called his military and national security advisers for a meeting at the White House to discuss the next steps. There were reports yesterday that US naval forces were moving closer to Syria, with the fourth ballistic-missile warship sent to the Mediterranean.

"The Defense Department has a responsibility to provide the President with options for contingencies, and that requires positioning our forces to be able to carry out different options – whatever options the President might choose," the US Defense Secretary, Chuck Hagel, said yesterday.

A White House official said: "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. The President has directed the intelligence community to gather facts and evidence so that we can determine what occurred in Syria. Once we ascertain the facts, the President will make an informed decision about how to respond."

Tomorrow, defence chiefs from the UK, France and other countries will meet in Amman, Jordan, to discuss the crisis. Sources insisted that the meeting had been planned since June, but it gives defence chiefs the opportunity to better understand the situation within Syria, according to Whitehall sources, including maintaining regional stability. General Sir Nick Houghton, Chief of the Defence Staff, will attend the summit.

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After what happened in Iraq, President Obama is cautious about launching any form of action – from imposing no-fly zones and arming the rebels to a full-scale military assault. The Obama-Cameron talks came as Angela Kane, the UN's disarmament chief, arrived in Damascus yesterday, to pressurise the Assad regime to allow UN inspectors, already in Syria, access to the site of the alleged attack.

Rebel forces have accused the Assad regime of launching the chemical weapons attack in the suburbs of the capital on Wednesday. But the Syrian government denies responsibility and says the opposition is behind the assault on the outskirts of the city. Western powers and even Syria's ally Russia have urged the Assad regime to let in a team of UN inspectors to investigate the allegations. France and Britain, who have both signalled strongly that they support military action in Syria, have said they believe Assad's forces were responsible.

"I know that some people in the world would like to say that this is some kind of conspiracy brought about by the opposition in Syria," said the Foreign Secretary, William Hague, on Thursday. "I think the chances of that are vanishingly small and so we do believe that this is a chemical attack by the Assad regime."

For the first time, the Iranian President, Hassan Rouhani, acknowledged that chemical agents had killed people in Syria, though he didn't say who was responsible. Russia has blamed the rebels. Both Russia and Iran are staunch allies of Damascus.

Syrian state TV, widely seen as a propaganda machine for the Assad regime, yesterday pinned the blame firmly on the rebels by airing footage of gas masks and other explosives apparently found in rebel territory.

"You've got to have some sort of process where you signal intent and this is first step", said Michael Stephens, a Doha-based researcher for the Royal United Services Institute, referring to the US naval build-up in the Mediterranean.

Chemical weapons

The state-controlled Syrian Arab News Agency claimed yesterday that Syrian army troops had uncovered a cache of chemical weapons in a network of tunnels used by rebel forces.

The soldiers, it said, "suffered from cases of suffocation" when rebels used "chemical agents" against them in the Damascus suburb of Jobar, close to where more than 1,000 civilians are reported to have been killed in a chemical attack last week.

The reports said the rebels had attacked "as a last resort" after government forces made "big gains". Television footage showed five blue and green plastic drums lined against a wall, as well as several rusty mortar bombs and grenades. Gas masks were shown, placed next to vials labelled "atropine", a nerve agent toxicity antidote. The presenter insisted that the images were proof that the rebels had used chemical weapons, but did not say which of the items contained them.

Critics claimed the reports were designed to deflect blame away from the Syrian government for a nerve gas attack in Damascus on Wednesday.

The international medical humanitarian organisation Doctors Without Borders said yesterday that three hospitals it supports in Damascus reported receiving 3,600 people displaying neurotoxic symptoms within three hours on Wednesday. Of those, 355 died. MSF says it cannot "scientifically confirm" the use of chemical weapons.

Tom Foot