UK Foreign Secretary William Hague says he believes President Assad was behind a chemical attack in Syria.
"I know that some people in the world would like to say that this is some kind of conspiracy brought about by the opposition in Syria," said Mr Hague.
"I think the chances of that are vanishingly small and so we do believe that this is a chemical attack by the Assad regime."
It was "not something that a humane or civilised world can ignore".
Pressing for UN weapons inspectors to be given access to the site, the UK foreign secretary said: "It seems the Assad regime has something to hide.
"Why else have they not allowed the UN team to go there?"
The UK had not ruled out any options for the future, added Mr Hague.
"Any option that complies with international law and could save innocent lives, we have to be open to those options," he said, but added any decisions would come later and he would not speculate about them.
US President Barack Obama has said the alleged use of chemical weapons in Syria in an attack on Wednesday is a "big event of grave concern".
He said the US was still seeking confirmation such weapons were used, but if proved true the situation would "require America's attention".
Speaking at the Foreign Office, Mr Hague warned that the UK would be ready to go back to the United Nations Security Council to secure a stronger mandate "for the world to speak together more forcefully about this" if there was no movement over the next few days.
He said he had spoken to UN Secretary General Ban Ki-moon this morning and hoped to speak to the Russian foreign minister later. He spoke on Thursday evening to US Secretary of State John Kerry.
Mr Hague said: "We need to make sure the world knows the facts of what has happened and that means the UN team that is in Damascus - only 20 minutes travel away - being able to get there and to investigate."
He added: "This is what we are focused on and we are working with countries all over the world to try to bring this about and to try to establish the truth to the satisfaction of the world about what is clearly a terrible atrocity.
"The only possible explanation of what we have been able to see is that it was a chemical attack and clearly many, many hundreds of people have been killed, some of the estimates are well over 1,000.
"There is no other plausible explanation for casualties so intense in such a small area on this scale."
Shadow foreign secretary Douglas Alexander called for a fresh UN resolution on Syria, saying its response so far was "clearly insufficient".
"The British government should now initiate steps with our allies on the UN Security Council to secure the agreement of a fresh resolution which not only condemns the use of chemical weapons but specifies the mandate of the UN inspection team already in Syria to include East Ghouta.
"Analysis and verification will rely on a swift and thorough inspection on the ground, so there is a real urgency to the UN team being granted full access by the Syrian authorities to the sites of the alleged attacks."