US Secretary of State John Kerry says that an international meeting on the Syria crisis had agreed to implement a "cessation of hostilities" in Syria and to immediately expand delivery of humanitarian aid.

Kerry, speaking after marathon talks that included Russia and more than a dozen other countries, said that all nations involved in the talks agreed that Syrian peace negotiations should resume in Geneva as soon as possible.

"We believe we have made progress on both the humanitarian front and the cessation of hostilities front, and these two fronts, this progress, has the potential -- fully implemented, fully followed through on, to be able to change the daily lives of the Syrian people," Kerry said.

Kerry, flanked by Russian Foreign Minister Sergei Lavrov and UN envoy Staffan de Mistura, acknowledged that the Munich meeting produced commitments on paper only. He and Lavrov agreed that the "real test" will be whether all parties to the Syrian conflict honour those commitments.

British Foreign Secretary Philip Hammond said a cessation of hostilities can only succeed if Russia stops air strikes supporting Syrian government forces' advance against the opposition.

"If implemented fully and properly, this (deal) will be an important step towards relieving the killing and suffering in Syria," Hammond said in a statement.

"But it will only succeed if there is a major change of behaviour by the Syrian regime and its supporters.

"Russia, in particular, claims to be attacking terrorist groups and yet consistently bombs non-extremist groups including civilians. If this agreement is to work, this bombing will have to stop: no cessation of hostilities will last if moderate opposition groups continue to be targeted."