Military action against Syria may be the only remaining response to the suspected large-scale chemical weapons attack by the Assad regime, William Hague said today.
The Foreign Secretary declined to be drawn on the options being considered by Western allies but would not rule out the possibility of air strikes or other measures being taken within days.
And he told MPs demanding the recall of parliament from its summer break ahead of any British involvement in any such intervention that it would "depend on the timing and nature of what we propose to do".
United Nations inspectors are due to visit the area of Damascus where chemical weapons were apparently deployed last week after the regime gave permission and a temporary ceasefire was agreed.
Syrian president Bashar Assad says the claims are "politically motivated" and defy logic as the regime has forces near the area - and warned in a Russian newspaper that any US-led military action would end in failure.
Mr Hague insisted there was "no other plausible explanation" and accused Damascus of delaying the arrival of the UN team to reduce the chances of them finding evidence.
Prime Minister David Cameron was involved in a round of phone calls with fellow leaders including American president Barack Obama over the weekend, who agreed the need to take "strong action".
The National Security Council is due to meet amid mounting speculation that the US, Britain and France could back limited airstrikes to show that deployment of chemical weapons will not be tolerated.
Mr Hague told BBC Radio 4's Today: "Obviously I can't go into the military options at the moment.
"The Prime Minister and President Obama... have agreed that there should be a strong response from the international community.
"We, the United States, many other countries including France, are clear that we can't allow the idea in the 21st century that chemical weapons can be used with impunity."
Pressed on whether action could be taken as early as this week - ahead of parliament's scheduled return on Monday - he said: "I am not going to rule anything in or out. I am not going to speculate about that in public."
Mr Hague said diplomatic efforts to resolve the bitter civil war in Syria had failed - and suggested military action could now be the only alternative to allowing chemical weapon use with impunity.
"This may be the choice. This is why we have called for a strong response".
Any intervention would be "in accordance with international law and will be based on legal advice to the national security council and to the cabinet", he stressed.
But it could be taken "without complete unity on the UN Security Council", he added, amid frustration over the continued support for the regime from Russia which has blocked previous efforts to secure UN backing.
Action was allowed "based on great humanitarian need", he suggested.
Mr Cameron is under pressure from the Labour Opposition and from Tory backbenchers to explain himself to MPs before intervening.
The Foreign Secretary said: "We have a good record on consulting parliament and having a vote in parliament if we decide to take any military action.
"Of course we are conscious of the views of parliament on these matters and the need to be consulted so we are very conscious of that.
"But our decisions on that will depend on the timing and nature of what we propose to do."
Downing Street says summoning the Commons from its summer break has not been ruled out - but stressed Mr Cameron "reserved the ability to take action very swiftly if needed".
Shaam News Network image purporting to show mourners next to bodies of victims of an attack
Mr Hague criticised the level of access granted to UN inspectors by the regime - which the US said showed it had "something to hide".
"We called for unimpeded and immediate access. That has not happened until now and even now it is not unimpeded; it is for a certain number of hours organised with the Assad regime.
"And we believe that if they thought somebody else had done it... Then they would have granted that immediate and unimpeded access straight away."
He went on: "There is no plausible explanation other than that it has been carried out by the Assad regime.
"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 we have no evidence they have, delivered by artillery or airpower that they do not possess, killing hundreds of people in areas already under their control.
"That is just not a credible explanation."
Medecins Sans Frontieres said hospitals it supports in Syria treated some 3,600 patients with "neurotoxic symptoms" on Wednesday, 355 of whom died.
It said the patients had arrived in three hospitals in the Damascus area. Staff described people suffering from convulsions, extreme salivation, contracted pupils and sight and respiratory problems.
Syrian state media has blamed rebel forces for the release of chemical agents, saying some government soldiers had been suffocated during fighting in the suburb of Jobar.
Assad told Russia's Izvestia the claims his regime used chemical weapons were "nonsense".
"How can the government use chemical weapons, or any other weapons of mass destruction, in an area where its troops are situated?" he said.
"This is not logical. That's why these accusations are politically motivated, and a recent string of victories of the government forces is the reason for it."
The weekend's intense round of diplomacy on the crisis was set to continue after the Prime Minister discussed the situation with Mr Obama for 40 minutes and ordered officials to examine "all options".
Obama and Cameron have been discussing the possibility of joint military action against the Syrian regime
He also spoke with French president Francois Hollande and German chancellor Angela Merkel from Cornwall, where he has been on a family holiday.
French foreign minister Laurent Fabius said the nature of the response "will be negotiated in coming days" but that it would be "proportionate".
In an echo of Mr Hague's stance, he added: "The only option that I can't imagine would be to do nothing."
Mr Obama previously suggested that the use of chemical weapons in Syria would be a "game changer".
But he has stopped short of committing to military intervention in the troubled country - a step that would risk a confrontation with Russia.
Mr Hague said the UN Security Council had failed to "shoulder its responsibilities" on Syria - and could have helped end the conflict "a long time ago" had Russia and China not deployed vetoes.
"Is it possible to respond to chemical weapons without complete unity on the UN Security Council? I would argue yes it is - otherwise it might be impossible to respond to such outrages, to such crimes, and I do not think that is an acceptable situation.
"It is possible to take action based on great humanitarian need and great humanitarian distress. It is possible to do that under many different scenarios.
"But anything we propose to do on this, the strong response we have talked about whatever form that takes, will be subject to legal advice, must be in accordance with international law."
The UK's thinking was "exactly the same" as the US and France, he insisted, playing down the stronger speculation in the British media of a strike happening within days.
Tory backbencher Andrew Bridgen said Parliament must be recalled if there is a prospect of action.
"Let them persuade us of what we could do that's going to make it actually better rather than just a knee-jerk reaction. Just being seen to do something is not always the answer. It's got to be constructive," he said.
Shadow foreign secretary Douglas Alexander said: "If...the Prime Minister is now considering military options involving UK personnel, then of course I would expect him to seek a recall of Parliament and to come to the House of Commons and make his case in advance of a decision being made."
Douglas Alexander wants a debate in parliament ahead of any air strikes
A Downing Street spokesman stressed no decisions had been taken and there was no timetable for action.
"The Prime Minister has made clear that MPs should have the opportunity to debate issues like this and that still stands, although we must reserve the ability to take action very swiftly if needed," the spokesman said.