President Bashar al-Assad cannot achieve a military victory in Syria's brutal civil war, despite recent military gains by the regime, Foreign Secretary William Hague has said.
Following talks in London of the 11-nation Friends of Syria group, Mr Hague said that only a "political solution" could bring an end to the three-year conflict.
He said that the group had agreed to step up support to the moderate opposition National Coalition as the only viable alternative to the regime or the Islamist extremists.
"Clearly we have to now assess our strategy based on the situation getting worse in Syria," he told a news conference at the Foreign Office.
"There will not be a military victory for any side - including by the regime.
There may be some who believe they can win a military victory but that is not available to any side.
"That is why it has always been clear that there has got to be a political solution in the end.
"That analysis remains the same.
"We have to step up our efforts to make sure that a moderate opposition stays in being, that it is able to function.
"Otherwise, Syrians face a choice between the Assad regime and extremists."
Mr Hague said that Britain was providing an additional £30 million in practical support to the moderate opposition forces, with the National Coalition office in London being granted the status of mission "in recognition of the strength of our partnership".
The UK will also join other countries in stepping up humanitarian assistance to areas not being reached by the UN, including by funding cross-border aid, with £76 million allocated by the UK to such operations.
Mr Hague said Britain would "strongly support" a new United Nations Security Council resolution to compel the regime to admit humanitarian aid and to halt its "starvation tactics", as well as the principle of a resolution referring the Damascus government to the International Criminal Court.
Mr Hague said: "We've had a strong and united and purposeful meeting which involves the stepping up of support to the moderate opposition in Syria and concerted action at the United Nations.
"We will work over the coming weeks to implement these commitments.
"We will intensify how closely we work together, given the deepening crisis in Syria, because we are all determined to respond to it."
Among those attending the meeting at the Foreign Office was US Secretary of State John Kerry, as well as representatives of Egypt, France, Germany, Italy, Jordan, Qatar, Saudi Arabia, Turkey and the United Arab Emirates.
The lack of progress in ending the three-year conflict that has killed at least 150,000 was underlined this week by the announcement of the resignation of the United Nations/Arab League special envoy Lakhdar Brahimi.
UN Secretary General Ban Ki-moon said the "hopelessly divided" international community was in part to blame for leaving Mr Brahimi battling "almost impossible odds" in brokering a deal between the Assad regime and increasingly fractured rebel elements.
In its final communique, the group condemned Assad's plan to stage "illegitimate" presidential elections on June 3.
"This mocks the innocent lives lost in the conflict ... and is a parody of democracy," it said.
It promised to step up efforts to deliver humanitarian aid "across borders and across lines irrespective of the consent of the regime".
Mr Hague said the meeting had also agreed to draw up plans to deliver humanitarian aid "across borders and across lines, irrespective of the wishes of the regime".
It comes amid pressure on the United Nations to drop its insistence that the consent of Damascus is required for aid operations and ensure help gets to up to 3.5 million people in "hard-to-reach" areas via neighbouring states.
The Foreign Secretary said the UK had already allocated £76 million to cross-border operations and would step up efforts to access " areas not being reached by the UN".
Support would also be given to a stronger UN Security Council resolution to get over the legal barriers, he added.
Mr Kerry said the US was "open to the idea of delivering aid by any means that will get it to the people who need it" and would "address this issue in every way possible".
He also said he had seen as-yet-unconfirmed evidence that the Syrian regime had used chemical weapons, in breach of its international treaty obligations.
French foreign minister Laurent Fabius has accused Bashar Assad's forces of at least 14 such offences since October.
But no concrete evidence has yet been produced to back suspicions of the use of chlorine in an attack on rebel areas last month.
"I have seen the raw data that suggests, as France is suggesting, that there may have been a number of instances in which chlorine has been used in the conduct of war," Mr Kerry said.
"It has been made clear by President Obama and others it would result in consequences. We are not going to pin ourselves down to a precise timetable but there would be consequences were it to be proven."
Asked about calls from the moderate opposition for the supply of arms, including surface-to-air missiles, he said: "Out of today's meeting, every facet of what can be done is going to be ramped up. Every facet. That includes political effort, it includes aid to the opposition, it includes economic efforts, sanctions.
"I'm not going to discuss specific weapons systems, except to say that every possible avenue that is available is going to be pursued by one country or another."
The UK and US assistance is restricted to "non-lethal" items.
"They will be ramped up to make it clear that despite the fact that Assad might think today he is doing better and that somehow this process is going to come to a close with him sitting pretty, the answer is 'no', we're not going away, the opposition is not going away." Mr Kerry said.