After an ambitious deal to end hostilities in Syria within a week was signed early on Friday,
doubts emerged over its viability as it excludes the Islamic State group and Al-Qaeda's local branch.
US Secretary of State John Kerry admitted there were "no illusions" about the difficulty of implementing a nationwide "cessation of hostilities" between regime forces and rebels as
he announced the deal in Munich alongside Russian Foreign Minister Sergei Lavrov.
The 17-nation International Syria Support Group also agreed that "sustained delivery" of humanitarian aid will begin "immediately", with a new UN task force meeting later in
Geneva to start pushing for much greater access to "besieged and hard-to-reach areas".
The deal, struck in late night talks in Munich, went further than expected, with Lavrov talking about "direct contacts between the Russian and US military" on the ground, where the powers are backing opposing sides in the five-year-old conflict.
But after a fortnight in which the forces of Syrian President Bashar al-Assad have besieged the key rebel city of Aleppo with the help of heavy Russian bombing, several nations put the onus on Moscow to implement the Syria deal.
"Through its military action on the side of Assad's regime, Russia had recently seriously compromised the political process. Now there is a chance to save this process," foreign ministry spokeswoman Christiane Wirzt said.
"What is important now is embracing this opportunity, stopping the airstrikes, ceasing targeting civilians and providing humanitarian access," added Turkish Foreign Minister
Mevlut Cavusoglu on Twitter.
Analysts remained sceptical about the chances of ending the bloodshed.
"(The agreement) is ambitious and yet very tenuous...there are huge question marks," said Julien Barnes-Dacey, of the European Council on Foreign Relations.
The failure to include Al-Qaeda affiliate Al-Nusra in the cessation of hostilities was particularly important, he said, since the group is active in Aleppo and surrounding regions,
and many of the more "moderate" rebels have links with it.
"In many ways this Munich meeting was thrust to the fore by the situation in Aleppo, and yet the conditions of the agreement do not seem to apply to Aleppo," said Barnes-Dacey.
"Talking about Nusra works in the Russians' favour since so many rebel groups have ties to Nusra. This effectively gives the green light for the Syrian government and its allies to carry on military action while paying lip service to the agreement."
Lavrov underlined that "terrorist organisations" such as IS and Al-Nusra "do not fall under the truce, and we and the US-led coalition will keep fighting these structures". But other analysts struck an optimistic note, saying it was significant that the US and Russia had been able to strike a deal at all.