The British Foreign Secretary William Hague has said for the first time that the "atrocity" in Syria "was a chemical attack" carried out by Assad forces.
Mr. Hague said that the chances of the attack being a conspiracy by opposition forces was "vanishingly small" and that the only "plausible explanation" for the mass deaths was an attack by the regime.
He said it was essential to get UN inspectors already in the country to the site outside Damascus and that "time was of the essence".
If inspectors were not allowed in, the evidence, he said, would deteriorate.
Forces loyal to President Bashar al Assad have been carrying out air strikes on the area, which could already have considerably damaged any proof of the use of nerve gas.
And Mr. Hague warned the Assad regime that if they did not comply with the UN request to carry out inspections at the scene of the attack then Britain would be prepared to go back to the UN Security Council for a mandate "for the world to speak more forcefully about this".
UN chemical weapons inspectors have been in the country since Sunday on a mission to investigate sites where previous gas attacks are alleged to have been carried out.
Activists said on Friday they were smuggling body tissue samples from victims to the UN inspectors in their hotel but that it had been a struggle because they were heavily guarded by government forces.
In his first comments since the footage of the attacks emerged, US President Barack Obama said it was a "big event of grave concern" and that " this is something that is going to require America's attention".
While the UN Secretary General Ban Ki-moon said that the use of chemical weapons would constitute "a crime against humanity".
He is asking the Syrian government to allow a team of United Nations experts to investigate the latest allegations regarding the use of chemical weapons and is sending one of his senior officials to Damascus in this regard.
"The Secretary-General believes that the incidents reported yesterday need to be investigated without delay", Mr. Ban's spokesperson told journalists in New York, adding that the UN chief remains deeply troubled about the alleged use of chemical weapons in Syria.
A UN team is currently in Syria spending up to 14 days, with a possible extension, probing the alleged use of chemical weapons by the Government at Khan al-Asal, as well as two other allegations reported by Member States.
UN Secretary-General is sending a High Representative to Damascus
Mr. Ban is requesting that the team, led by Swedish scientist Åke Sellström, be granted permission and access to 'swiftly' investigate the incident .
Significantly, Syria's key ally Russia has joined international calls for the inspectors to be given access to the site of the alleged massacre. Moscow suggested the attack could be a "premeditated provocation" by opposition forces but urged Mr Assad and the UN to agree to a visit to the site in the Damascus suburbs of Ein Tarma and Zamalka.
