Unanimous approval of a United Nations resolution endorsing a peace process for Syria is a "significant step" towards ending the country's civil war, Foreign Secretary Philip Hammond has said.
But he accepted that there was "still a very long way to go" and the text failed to address what role Bashar Assad should play - one of the major diplomatic sticking points.
In a rare show of international unity over the conflict, Russia backed the resolution endorsing the start of "urgent" formal negotiations between President Assad's regime and moderate opposition groups early next month.
It says it should establish "credible, inclusive and non-sectarian governance" followed within 18 months by UN-supervised "free and fair elections" with a parallel effort to secure a ceasefire.
Any halt to hostilities would not apply to air strikes against so-called Islamic State (IS) - also known as Isis, Isil and Daesh - and other terrorist groups, it makes clear, amid fears they could otherwise benefit.
A peace plan agreed last month by 20 nations meeting in Vienna sets a deadline of January 1 for the start of negotiations.
US secretary of state John Kerry, who joined Mr Hammond and other foreign ministers for talks among the International Syria Support Group (ISSG) in New York, said that by setting out clear goals and timings, the resolution was a clear "milestone".
Addressing the UN meeting, Mr Hammond said: "Sadly it is far too soon for any of us to predict an end to the Syria conflct.
"But I hope that we will look back on today as a significant step in that direction."
Demanding an end to repeated failures to secure UN agreement over the way forward, he said the international community "have to do better fast if there is not to be still more suffering".
Russian support for Assad has been the main obstacle to international unity, with President Vladimir Putin insisting the long-standing protege of Moscow should be allowed to stand in elections to choose any new administration.
Britain remains insistent that Assad can remain in place temporarily as part of a transitional administration, but cannot have a long-term role in government.
Mr Hammond said: "This process necessarily involves the departure of Bashar al Assad, not only for moral reasons because of the destruction that he has unleashed on his own people, but also for practical reasons because it will never be possible to bring peace and unity to Syria as long as he remains in office.
"But we must and will protect the institutions that are necessary for the future governance of Syria and that will be possible with a representative transitional governing body and with the support of the ISSG."
There is also disagreement over which opposition groups should form part of the negotiations.
Mr Hammond took a swipe at Russia over what the West says is its deliberate targeting of moderate Syrian opposition forces in air strikes.
"It is vital that all countries that claim to be fighting Daesh do what they say rather than directing the bulk of their attacks against non-extremist opposition groups," he said.
"There is clear evidence over the last weeks that the weakening of such groups has created opportunities for the expansion of Daesh in certain areas, the very opposite of the stated objectives.
"The conflict in Syria is now almost five years old. In that time more than 250,000 Syrians have been killed. We all have a duty to prevent future slaughter.
"Despite the important step that we have taken with today's resolution, despite the progress we have made in Vienna, despite the important steps forward taken at the meeting in Riyadh, there is still a very long way to go."