British Foreign Secretary William Hague on Sunday said that any evidence of a chemical attack by the Syrian regime may have already been destroyed.

"The fact is that much of the evidence could have been destroyed by that artillery bombardment," he said, referring to reported continued attacks on the area east of Damascus where the chemical attack is believed to have taken place.

"Other evidence could have degraded over the last few days and other evidence could have been tampered with," he added, during a press conference given shortly after Damascus gave its green light to a mission by UN inspectors.

The experts are Monday to start investigating the site of the alleged attack as a sceptical Washington said Syria's acceptance had come too late.

Hague expressed concern that too much time had elapsed for the UN inspectors to gather enough concrete evidence.

"We have to be realistic now about what the UN team can achieve," he said.

However, he repeated his belief that Syrian President Bashar al-Assad's forces were responsible for the alleged attack, claiming "there is a lot of evidence already and it all points in one direction."

"We are clear in the British government that it was the Assad regime that carried out this large scale chemical attack," he added.

"The eye witness accounts, the fact this area was under bombardment by the regime forces at the time that the chemical attack took place.

"If the regime believed somebody else had carried out this attack then they would have given access to the UN inspectors several days ago," he argued.

The minister said Britain was working with the international community to formulate a response, with Prime Minister David Cameron and US President Barack Obama talking via telephone on Saturday.

"They are agreed there must be a serious response by the international community," he explained.

"We cannot, in the 21st century, allow the idea that chemical weapons can be used with impunity, that people can be killed in this way and that there are no consequences for it."

He would not outline possible responses "for obvious reasons", but stressed it was "very important to act in accordance with international law and... to have widespread international support".

More than 100,000 people have been killed in Syria since an uprising against Assad's rule flared in March 2011, the UN says.