Foreign powers have been searching for a response since hundreds of people were killed by poisonous gas last Wednesday in the suburbs of Damascus.
The UN yesterday said Damascus had agreed to a ceasefire while its experts are at the site for inspections which were to begin today. Syria confirmed it had agreed to the inspections.
Crucially, it is understood the UN team would be able to confirm the use or not of chemical weapons, but has no remit to report on who used them.
Foreign Secretary William Hague said last night evidence could have been destroyed or tampered with since the attack.
The eastern Ghouta area where the attack took place is under opposition control, making arranging a trip across the front lines difficult. Rebels and the main Western-backed opposition group have said they would guarantee access and the safety of the UN team.
There are increasing signs that the US and its allies are considering taking action, a year after president Barack Obama said the use of chemical weapons was a “red line” that would prompt serious consequences.
A senior US official said there was little doubt the Syrian government had used a chemical weapon against civilians on Wednesday, and Washington was weighing its response.
“At this juncture, any belated decision by the regime to grant access to the UN team would be considered too late to be credible, because the evidence available has been significantly corrupted as a result of the regime’s persistent shelling and other international actions over the last five days,” the official said.
Syria’s information minister said any US military action would “create a ball of fire that will inflame the Middle East”.
He said Damascus had evidence chemical weapons were used by rebels fighting to topple president Bashar al-Assad, not by his government. Western countries say they believe the rebels do not have access to poison gas.
Western leaders have been calling one another in recent days, and issuing declarations promising a response.
“This crime must not be swept under the carpet,” Prime Minister David Cameron’s office said after a telephone call with French president Francois Hollande about the crisis yesterday morning. Mr Hollande’s office said. “France is determined this act does not go unpunished.”
A team of UN chemical weapons inspectors had arrived in Syria three days before the incident to investigate earlier reports of chemical weapons use. Since Wednesday, the 20-strong team has been waiting in a hotel a few miles from the site of what appears to have been the world’s worst chemical weapons attack since Saddam Hussein’s forces gassed thousands of Iraqi Kurds in 1988.
State TV showed tanks moving on yesterday into what it said was the eastern Damascus suburb of Jobar, one of the districts where the mass poisoning occurred. Opposition activists in Damascus said the army was using surface-to-surface missiles and artillery in the area.
Mr Obama met military and national security advisers on Saturday to discuss options. US naval forces have been repositioned in the Mediterranean to give him the option of a military strike. And the US Navy has sent a fourth warship armed with ballistic missiles closer to Syria.
“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 US intelligence community, and international partners, there is very little doubt that a chemical weapon was used by the Syrian regime against civilians in this incident,” the US official said.
Assad’s closest ally Iran, repeating Mr Obama’s own rhetoric, said the US should not cross a “red line” by attacking Syria. “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,” said Massoud Jazayeri, deputy chief of staff of Iran’s military .
Russia, Mr Assad’s UN Security Council ally which has suggested rebels may have been behind the attack, welcomed the decision to allow the UN investigation and said it would be a “tragic mistake” to jump to conclusions over who was responsible.
In past incidents, the US, Britain and France have said they obtained their own proof Mr Assad used small amounts of chemical arms. If the UN team obtains independent evidence, it could be easier to build an international diplomatic case for intervention.
The Obama administration is reluctant to be drawn deep into another war in the Muslim world after pulling US forces out of Iraq and preparing to withdraw from Afghanistan. Democrat Senator Jack Reed yesterday said any US response had to have international military support.
The Syrian opposition says well over 1,000 civilians died on Wednesday. The medical charity Medecins Sans Frontieres said three hospitals near Damascus had reported 355 deaths in the space of three hours out of about 3,600 admissions with neurotoxic symptoms.