Foreign Secretary William Hague urged both sides in Syria to "seize the chance" to end the civil war as he arrived in Switzerland for the start of peace talks.
Parties are gathering for the biggest diplomatic push yet to halt the bloodshed which has cost more than 100,000 lives and sparked a massive humanitarian emergency.
A last-minute pitfall was avoided when a United Nations invitation to Iran to attend - over which the main Western-backed oppositon said it would boycott the talks - was withdrawn.
Tehran, a key ally of the regime of Syrian president Bashar Assad, was told it was no longer welcome after refusing to endorse a UN-backed plan for a transitional governing body,
"Opposition has been tested and has come. Now regime must be tested on willingness to seek a political solution," Mr Hague wrote on Twitter after arriving in Montreux.
"Both sides should seize chance to end the war. 100,000-plus Syrians have died in 18 months since Geneva I."
The so-called Geneva II process is set to get under way however with a renewed focus on alleged large-scale torture and execution of prisoners by government forces.
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.
Mr Hague said there was " compelling and horrific" evidence of abuses and that those responsible should be held to account.
Damascus denied allegations of abuses after the emergence of the gruesome images - apparently smuggled out by a defector from the regime.
The report into them was commissioned by Qatar, which supports Syrian rebel and opposition groups.
Mr Hague said it was a "great shame" that Iran - which has enjoyed a thawing of relations with the West in recent months - had failed to endorse the principles of the talks.
It was very important that there should be an " Iranian commitment" to any peace deal, he told MPs - which was why he had not been opposed to Tehran's involvement in principle.
In a telephone call with Iranian counterpart Mohammad Javad Zarif, Mr Hague insisted Britain "remained open" to working with Tehran to end the conflict.