Chemical weapons are the only "plausible explanation" for the deaths of 1,300 people in an attack outside Damascus, William Hague said on Friday, refusing to rule out any options for the West to prevent further killing.
Speaking at the Foreign Office, 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.
"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," he said.
"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."
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.
"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."
US president Barack Obama said officials were gathering information about the attack which is said to have involved rockets loaded with toxic agents, telling CNN: "What we've seen indicates that this is clearly a big event of grave concern. It is very troublesome."
"We don't expect co-operation (from the Syrian government), given their past history," the president added.
Asked whether the US government is facing a "more abbreviated time-frame" on key decisions in Egypt and Syria, the president replied "yes".
"We have to think through strategically what's going to be in our long-term national interests," he said.
Russia, which has vetoed past attempts to secure a tough UN resolution and suggested the attack could be "premeditated provocation" by opposition forces, has called for UN experts to be allowed to inspect the attack site.
Russia's foreign minister Sergey Lavrov and Kerry agreed they had a "mutual interest" in calling for the UN investigation during a telephone conversation, a statement released by the Russian interior ministry said.
Britain formally wrote to Ban yesterday with 36 other countries, calling for the UN team to be given access to the site.
A team of UN weapons inspectors is in Syria but only has permission to visit specific locations. The regime has dismissed as "baseless" claims it was behind the latest incident.
Unverified footage of casualties, including children, in makeshift hospitals suffering convulsions and breathing difficulties has circulated on YouTube.
Accounts of the death toll vary wildly from around 100 to the claim by the Syrian National Coalition, the main opposition group in exile, that 1,300 were killed. The group said it was basing its claim on accounts and photographs by activists on the ground.
Syria is thought to have some of the world's largest stocks of chemical weapons, including mustard gas and the nerve agent sarin, but the government in Damascus refuses to confirm it.
Defence Secretary Philip Hammond told the BBC: "Of course the evidence is not yet corroborated but we all know what we have seen on our television screens.
"We will press the regime in Damascus to give proper access to the United Nations inspectors so we can establish exactly what has happened.
"If, as the evidence suggests, chemical weapons have been used on a significant scale against a civilian population, those who are responsible should be in no doubt they will be held to account by the international community."
Hans Blix, who led UN weapons inspectors in Iraq, said the work of the team in Damascus would be absolutely vital to ensure the world had "objective data" as it responded to the crisis in Syria.
He told the BBC Radio 4 World at One programme the problems faced by inspectors in Damascus were worse than he had faced in Baghdad ahead of the 2003 Iraq war. "I don't think they can just put themselves in a jeep and drive to the site," he said.
"Here is fresh evidence but (the team) know that it is part of a much larger play.
"As in any criminal inspection, the sooner you are on the site, the better."
The practice of inspections and evidence gathering was vital, Blix added.
"This is how the international community must develop," he said.