Syria has agreed to allow UN inspectors access to sites in suburbs of Damascus where alleged chemical attacks occurred on Wednesday.

The concession follows accusations that Bashar Assad's regime is responsible for a chemical attack which killed more than 350 people - many of them children - on Wednesday. 

Syria has denied that it is to blame while many Western officials claim the poisoning was caused by a chemical agent used in a rocket attack carried out by government forces.

A statement released by the Syrian government said: 'The Syrian government and the United Nations agreed on a common understanding ... to allow the United Nations to investigate allegations of chemical weapons use in the Damascus suburbs.'

Many hundreds of people were poisoned to death on Wednesday before dawn in what appears to have been the world's worst chemical weapons attack since Saddam Hussein's forces gassed thousands of Iraqi Kurdish villagers in 1988.

The incident took place just three days after a UN chemical weapons team arrived in Syria to investigate other smaller allegations of poison gas use. The inspector team's movements must be agreed with the Syrian authorities.

The Syrian statement said that the date and time of the inspectors' visit to the site had been agreed, but it did not say when it would take place.

Syrian Foreign Minister Walid al-Moualem met UN High Representative for Disarmament Affairs Angela Kane - who was in Damascus to negotiate access - this morning.

Moualem stressed 'Syria's readiness to cooperate with a team of investigators to uncover false allegations by terrorist groups that Syrian troops used chemical weapons.'

U.S. President Barack Obama and top advisers are debating options for responding to the reported use of chemical weapons in Syria amid what Britain called 'increasing signs' that the Syrian government was responsible for the attack.

A senior U.S. government official told Reuters: 'Based on the reported number of victims, reported symptoms of those who were killed or injured, witness accounts, and other facts gathered by open sources, the U.S. intelligence community, and international partners, there is very little doubt at this point that a chemical weapon was used by the Syrian regime against civilians in this incident.

'We are continuing to assess the facts so the president can make an informed decision about how to respond to this indiscriminate use of chemical weapons.'

Meanwhile Iran has warned the United States it will face 'severe consequences' if it crosses the 'red line' on Syria.

Deputy chief of staff of Iran's armed forces Massoud Jazayeri said: 'America knows the limitation of the red line of the Syrian front and any crossing of Syria's red line will have severe consequences for the White House.'

President Obama has been holding talks with David Cameron over the growing crisis in Syria.

The Prime Minister and the U.S. President discussed the conflict by telephone last night and have ordered officials to examine 'all options'.

A Downing Street spokesman said the leaders spoke for around half an hour.

'They are both gravely concerned by the attack that took place in Damascus on Wednesday and the increasing signs that this was a significant chemical weapons attack carried out by the Syrian regime against its own people,' he said.

'The UN Security Council has called for immediate access for UN investigators on the ground in Damascus. The fact that President Assad has failed to co-operate with the UN suggests that the regime has something to hide.

'They reiterated that significant use of chemical weapons would merit a serious response from the international community and both have tasked officials to examine all the options.

'They agreed that it is vital that the world upholds the prohibition on the use of chemical weapons and deters further outrages. They agreed to keep in close contact on the issue.'

President Obama previously suggested that the use of chemical weapons in Syria would be a 'gamechanger'.

But he has stopped short of committing to direct intervention in the troubled country - a step that would risk inflaming tensions with Russia.

Mr Cameron would also face domestic resistance to military action, with Tory MPs already having insisted there should be a Commons vote before arms are supplied to rebels.

However, there is speculation that the U.S., Britain and France could back limited airstrikes to demonstrate that deployment of chemical weapons will not be tolerated.

Mr Cameron also spoke to Canadian PM Stephen Harper, who agreed that the 'international community must respond appropriately'.

Medecins Sans Frontieres said hospitals it supports in Syria treated some 3,600 patients with 'neurotoxic symptoms', 355 of whom died.

It said the patients had arrived in three hospitals in the Damascus area on Wednesday. Staff described people suffering from convulsions, extreme salivation, contracted pupils and sight and respiratory problems.

However, the organisation stressed it could not 'scientifically confirm' the use of chemical weapons.