Foreign Secretary William Hague has urged the warring sides in the Syrian civil war to "put an end to the devastation of your country" by staying at the negotiating table, as peace talks to end the three-year conflict opened in Switzerland.
Speaking at the "Geneva II" conference, Mr Hague called for a firm timetable for the creation of a transitional government acceptable to both sides, along with ceasefire agreements to allow full and immediate humanitarian access to the millions of people who have been forced from their homes by the fighting.
US secretary of state John Kerry told the conference there was "no way" that Syria's president Bashar Assad can form part of any future government for the Middle Eastern state.
Representatives of both the Assad regime and the main opposition groups have gathered in the town of Montreux for the biggest diplomatic push yet to halt the bloodshed which has cost more than 130,000 lives and sparked a massive humanitarian emergency.
Mr Hague urged both sides in Syria to "seize the chance" to end the civil war as he arrived at the talks, which are intended to bring about the implementation of a roadmap for peace drawn up in Geneva in 2012.
"The only way to end the bloodshed and deal with extremist threats is to reach an inclusive political settlement that takes into account the needs and aspirations of all Syria's communities, with a Syrian-led political process leading to a transition, enabling the Syrian people independently and democratically to determine their future," said Mr Hague.
"These talks are only the start of a process that will require commitment and courage, but I urge both sides to remain at the negotiating table.
"To them I say: this is your opportunity to put an end to the devastation of your country. Now is the time to choose to save a generation of Syrian children from violence and trauma, to end the siege being laid to ancient towns and cities, to begin to repair the rich fabric of Syrian society and to spare millions of refugees the prospect of years of exile, homelessness and deprivation.
"We have no illusions about how difficult and challenging this process is likely to be, but we should all do everything possible to help the people of Syria achieve peace."
Opening the talks, United Nations secretary-general Ban Ki-moon acknowledged that the delegates face "formidable" challenges and called on both the regime and opposition to negotiate in good faith.
"We know that it has been an extremely difficult path to reach this point. We have lost valuable time and many, many lives," said Mr Ban. "Let me not mince words, the challenges before you and before all of us are formidable. But your presence here raises hope."
Mr Kerry told the conference: "We have not only an opportunity but we have an obligation to find a way forward, so that the people of Syria can choose their leadership, know peace and - for nine million refugees - finally be able to return home in dignity."
But he made clear that the US believes that there can be no place in any transitional government for Assad, whom he blamed for killing and torturing his own countrymen. The only obstacle standing in the way of the "roadmap to peace" set out in Geneva was "the stubborn clinging to power of one man, one family", he said.
The transitional administration envisaged in the roadmap for peace cannot include anyone objected to by either side in the conflict, said the US secretary of state.
He added: "That means that Bashar Assad will not be part of that transitional government. There is no way possible in the imagination that the man who has led the brutal response to his own people could regain the legitimacy to govern.
"One man - and those who have supported him - can no longer hold an entire nation and the region hostage."
Mr Kerry said the thousands of deaths in Syria since the first peaceful demonstrations against the regime in 2011 had been carried out "almost exclusively" using weapons which were in the possession only of the Assad administration.
Photographs released in the last few days had shown "systematic torture and execution of thousands of prisoners" in what Mr Kerry described as "an appalling assault not only on human lives but on human dignity and on every standard by which the international community tries to organise itself".
Damascus has denied allegations of abuses after the emergence of the gruesome images - apparently smuggled out by a defector from the regime and published in a report commissioned by Qatar, which supports Syrian rebel and opposition groups.
But former war crimes prosecutors who examined more than 55,000 photographs said to show the emaciated corpses of victims say they implicate the "agents" of the Assad regime, while Mr Hague said there was "compelling and horrific" evidence of abuses and those responsible should be held to account.
Mr Kerry also said there could be no place in the post-war government for "the thousands of violent extremists who spread their hateful ideology and worsen the suffering of the Syrian people".
It was the Damascus regime's intransigence which had made Syria "a magnet for terrorists" and there was "no prayer of reducing the increase in terrorism" until the transition to a mutually-agreed administration takes place, he said.
The talks began only after the UN's last-minute withdrawal of an invitation to Iran to attend, which sparked warnings of a boycott by the main Western-backed opposition.
Tehran, a key ally of the Assad regime, was told it was no longer welcome after refusing to endorse the UN-backed plan for a transitional governing body,