Britain and the US are inching towards a military attack against the regime of Bashar al-Assad after William Hague said all other options have failed over the past year.

As the Syrian president said the US would face failure if it intervened in his country, the UK foreign secretary said Britain and its allies could intervene without the authority of the UN.

Hague, who insisted Britain shared a common position with the US and France, told BBC Radio 4's Today programme: "We have tried those other methods, the diplomatic methods, and we will continue to try those. But they have failed so far."

The foreign secretary admitted that Britain effectively faced a stark choice between inaction or a military strike as UN weapons inspectors embarked on a visit to the area east of Damascus that was struck by an apparent chemical weapons attack last week.

General Sir Nick Houghton, the chief of the defence staff, is to discuss military options with his US counterpart, General Martin Dempsey, and other allied military chiefs at a summit in the Jordanian capital of Amman.

Russia and China are likely to veto any UN security council resolution authorising military action, but Hague said such a move could be legal under international law even without UN approval.

"It is possible to take action based on great humanitarian need and humanitarian distress. It is possible to do that under many different scenarios," he said. "But anything we propose to do, the strong response we have talked about, whatever form that takes, will be subject to legal advice, must be in accordance international law."

The foreign secretary indicated that Britain's patience was wearing thin after all other forms of pressure on Syria had failed over the past year.

"Of course we want the maximum pressure from world opinion, from diplomatic work, on the Syrian regime not to do these things again. It has to be pointed out that such pressure does not appear to have worked," he said.

"We have discussed over the last year the smaller scale chemical attacks that the regime has carried over the last year. On every occasion we have given direct messages, sometimes passed to the UN, to the Syrians not to do that. We have discussed it with the Russians and indeed sometimes the Syrians have heard from the Russians and the Iranians that they should not conduct chemical attacks.

"This does not appear to have worked because here is a large-scale chemical attack for which there is no plausible explanation other than that it has been carried out by the Assad regime."

Asked by the Today programme presenter Evan Davis whether the only options were a military attack or doing nothing, Hague said: "This may be the choice. This is why we have called for a strong response. What has happened over the last year is that the Assad regime has got used to the idea of making small-scale attacks.

"Here, in an area east of Damascus that has been contested militarily for the last 12 months, the regime have been unable to make the gains on the grounds that they were looking for. This attack has taken place during a period when the regime was bombarding it.

"To believe that anybody else had done it you would have to believe that the opposition in Syria would use on a large scale weapons that we have no evidence they have, delivered by artillery or air power that they do not possess, killing hundreds of people in areas already under their control. That is just not a credible explanation."

Hague dismissed Assad's claims that his regime was not responsible for the chemicals weapons attack. He said: "The Assad regime did this. The use of chemical weapons in the 21st century, on a large scale like this, cannot go unaddressed, cannot be ignored. Our position is the same as France and the US."

Hague's comments came after Assad denied using chemical weapons. He told the pro-Kremlin newspaper Izvestia: "Would any state use chemical or any other weapons of mass destruction in a place where its own forces are concentrated? That would go against elementary logic."

Assad added that military action would fail. In remarks reported by Reuters in Moscow, he said: "Failure awaits the United States as in all previous wars it has unleashed, starting with Vietnam and up to the present day".