The Foreign Secretary is involved in a series of talks with key international figures over the next steps to take and stressed that “time is of the essence” in getting UN weapons inspectors to the scene.
Speaking at the Foreign Office, Mr Hague warned the UK will 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 is no movement over the next few days.
Mr Hague said: “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.
“I know some people in the world would like to say this is some kind of conspiracy brought about by the opposition in Syria - I think the chances of that are vanishingly small. So we do believe this is a chemical attack by the Assad regime on a large scale but we would like the United Nations to be able to assess that so those who don’t believe that, those who doubt that, the evidence can be gathered.
“But that is certainly our opinion.”
Mr Hague has held talks with US Secretary of State John Kerry and UN Secretary General Ban Ki-Moon and will continue discussions with other international leaders throughout the day.
He 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.
“The United Kingdom called the meeting of the UN Security Council on Wednesday night and Security Council members expressed their support for the UN team to go there.
“They have not yet been able to and already it seems the Assad regime has something to hide - why else have they not allowed the UN team to go there?
“Of course we hope they will be able to go there. I discussed this this morning with the UN secretary general Ban Ki-Moon, and he agreed time is of the essence, that he is pressing for the UN team to be able to gain unimpeded access to the site.
“He is sending other people to Damascus so I said he had our strong support in that and Britain will help in any way that we can with that.
“I am encouraging other countries to support this too. I hope to speak to the Russian foreign minister later today, I have discussed it with secretary Kerry of the United States last night, and with many other countries, including the Turkish foreign minister who was here yesterday and the Qatari foreign minister who will be here today.
“This is our priority at the moment: to make sure the UN team can investigate on the ground and establish the facts.
“If that does not happen though within some days, since time is of the essence in these things the evidence will deteriorate over a matter of days, then we will need to be ready to go back to the Security Council to get a stronger mandate and for the world to speak together more forcefully about this so there can be access.”