UN representative Angela Kane has arrived in Syria to push for access to a suspected chemical weapons attack site, as Bashar al-Assad’s regime was accused by Western leaders today of being responsible for the massacre of 1,300 people with chemical weapons.

Russia, the Syrian President’s strongest ally, asked him to co-operate with UN inspectors, but also claimed that the attack may have been carried out by the rebels.

It has emerged, in the meantime, that senior members of the US administration have met in the White House to discuss a possible response to the massacre in Ghouta, an eastern suburb of Damascus, with options including Tomahawk launches and a more sustained air campaign.

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US Defence Secretary Chuck Hagel said the Pentagon was "positioning our forces" ahead of "whatever options the president might choose" as its navy presence in the eastern Mediterranean was bolstered.

However, no immediate action is likely with divisions within the military and diplomatic leadership.

The chairman of the Joint Chiefs of Staff, General Martin Dempsey, remains staunchly opposed to armed intervention while others, including Susan Rice, the UN representative, had been pressing for action which would send an unequivocal message to the Damascus regime.

With more graphic and shocking footage appearing of the killings, the growing consensus was that chemical weapons had been used with the US and western European states holding President Assad responsible.

Barack Obama, who had in the past declared that the use of weapons of mass destruction would cross a “red line”, said, “What we’ve seen indicates that this is clearly an event of grave concern. This is something that is going to require America’s attention.”

William Hague, the Foreign Secretary, held: “The only possible explanation of what we have been able to see is that it was a chemical attack.

“So we believe this is a chemical attack by the Assad regime on a large scale… It was the only plausible explanation for casualties so intense in such a small area.”

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The odds that rebels had staged the attack to “frame” the regime, said the Foreign Secretary, were “vanishingly small”.

Mr Hague spoke on the phone today to the US Secretary of State, John Kerry, and the Russian Foreign Minister, Sergei Lavrov, as well as Ban Ki-moon, the UN Secretary-General, stressing that the inspectors must get access to the affected area from the regime as soon as possible.

Prolonged delay would make the collection of viable evidence impossible and, if that happens, the UK intends to take the issue back to the Security Council.

A UN spokesman said that Mr Ban was “deeply troubled” by reports of the alleged attack.

“He expects to receive a positive response without delay.”

The high representative for disarmament, Angela Kane, was on her way to Damascus to press the regime for inspectors to be given access to the affected areas.

The opposition said it was trying to move survivors out of the area to safer places where they could receive treatment.

The Russian foreign ministry claimed in a statement, meanwhile, “a homemade rocket loaded with an unidentified chemical agent” was used in the attack which was “probably a provocation” by the opposition to implicate President Assad.

But the Kremlin failed to provide any further details to back up the charge.

The Syrian Deputy Prime Minister, Qadri Jamil, saw a foreign hand at work, “since no Syrian can do this to each other”.

In their version of events, opposition activists said that the first rockets bearing nerve agents were fired from a bridge on the highway from Damascus to Homs and others were launched from the Sironex factory in the Qabun district of the capital.

The areas are under regime control.

Mr Lavrov, in a telephone call to Mr Kerry, acknowledged that it was in the interest of both the countries for an investigation to take place.

“It is now up to the opposition to ensure safe access for the mission to the site of the alleged incident,” he is reported to have stressed.