Prime Minister David Cameron will decide tomorrow whether to recall Parliament to discuss the possibility of military action against Syria, Downing Street has said.
The announcement came after UN weapons inspectors investigating an alleged chemical attack in Damascus were shot at by snipers.
The team's lead vehicle was "deliberately shot at multiple times" as they left a government checkpoint during a visit to part of the city where hundreds of people were allegedly killed, Martin Nesirky told Sky News.
The UN's Angela Kane leaves the inspectors' hotel in Damascus
None of the inspectors, who were dressed in body armour, are believed to have been injured in the attack, which came after two mortar bombs landed close to their hotel.
"The inspectors are determined to carry out the mission they are mandated to carry out," Mr Nesirky said.
"But it's obvious that all sides need to extend their co-operation so they can conduct this work safely."
Mr Nesirky said it was not clear who was responsible for the sniper attack, although Bashar al Assad's regime blamed the mortar fire on rebel fighters.
After the shooting, the UN inspectors were able to change vehicles and meet victims of the alleged chemical attack, taking samples for testing before returning to their hotel in Damascus.
UN inspectors speak to medics shortly after their convoy was attacked
UN Secretary-General Ban Ki-Moon did not blame either side for the convoy attack but said he had told his disarmament chief Angela Kane to make a "strong complaint" to the Syrian government.
The Assad regime said the inspections in Zamalka and Ein Tarma districts will prove allegations that chemical weapons were used against civilians, including children, were "lies".
The opposition claimed 1,300 people were killed, while Doctors Without Borders said 355 people died in hospital from "neurotoxic" symptoms.
Mr Ban said the people of Syria deserved to know the truth, adding: "We cannot allow impunity in what appears to be a grave crime against humanity."
The international community continues to consider its response to the crisis, with the Foreign Secretary warning military action may be the only remaining option.
Doctors Without Borders said 355 people died in the alleged attack
Hopes of a peace conference in Geneva appear to be receding, with sources in the Syrian opposition telling Reuters the talks are "off the table" after the alleged chemical attack.
William Hague said Britain and other countries could respond to the alleged attack without the unanimous backing of the UN, telling the BBC: "Otherwise it might be impossible to respond to such outrages, such crimes."
His comments came as David Cameron announced he was cutting short his holiday to chair a meeting of the UK's National Security Committee, which is expected to take place on Wednesday.
Deputy Prime Minister Nick Clegg has cancelled a planned trip to visit British troops in Afghanistan so he can attend the meeting.
Shadow foreign secretary Douglas Alexander urged the Prime Minister to recall Parliament to "make his case in advance of a decision being made".
A Downing Street spokesman insisted no decisions had been taken on military action and said there was no clear timetable.
Bashar al Assad denied his regime had used chemical weapons
Meanwhile, French politicians are preparing to meet "in the coming days" to decide whether to respond with force, according to the country's foreign minister Laurent Fabius.
There is mounting speculation that France, together with Britain and the US, could back limited airstrikes to demonstrate that deployment of chemical weapons will not be tolerated.
However, Mr Assad said military intervention by the US would fail.
"Failure awaits the United States as in all previous wars it has unleashed," the Syrian president said in an interview with Russian newspaper Izvestia.
Describing the chemical weapons accusations as "nonsense", Mr Assad added: "Would any state use chemical or any other weapons of mass destruction in a place where its own forces are concentrated? That would go against elementary logic.
"Accusations of this kind are entirely political and the reason for them is the government forces' series of victories over the terrorists."
Mr Assad received backing from two of his main allies, with Russia's Foreign Minister Sergei Lavrov warning that the use of force without a UN mandate would violate international law.
Urging the West not to go down a "dangerous path", he said: "If anybody thinks that bombing and destroying the Syrian military infrastructure, and leaving the battlefield for the opponents of the regime to win, would end everything - that is an illusion."
Meanwhile, Iran's Supreme Leader Ayatollah Ali Khamenei blamed countries outside the Middle East for the turmoil and said states in the region backing the Syrian opposition would be damaged by the conflict.
"The main reason behind the status quo in the region is interference from outside the region," the state news agency Fars quoted Khamenei as saying in a meeting with Oman's Sultan Qaboos.