British Foreign Secretary William Hague said Sunday 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 at a press conference held 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 said, citing "the eyewitness accounts (and) the fact this area was under bombardment by the regime forces at the time that the chemical attack took place."
The minister argued: "If the regime believed somebody else had carried out this attack then they would have given access to the UN inspectors several days ago."
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He said Britain was working with the international community to formulate a response, with Prime Minister David Cameron and US President Barack Obama talking by telephone on Saturday.
"They are agreed there must be a serious response by the international community," he said.
"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."
Hague 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".
Cameron's office later revealed that the prime minister had on Sunday spoken to German Chancellor Angela Merkel about the crisis.
"The prime minister called Chancellor Merkel to discuss how the international community should respond to the chemical weapons attack in Syria last week," a Downing Street statement said.
"They agreed that this was a very grave incident and that there was little doubt that it had been carried out by the regime, particularly given their refusal to grant the UN access to the site immediately after the attack."
The pair also agreed that such an attack "demanded a firm response from the international community."
More than 100,000 people have been killed in Syria since an uprising against Assad's rule flared in March 2011, according to the United Nations.