US Secretary of State John Kerry on Thursday demanded that Russia stop bombing the Syrian opposition, implicitly blaming Moscow for the collapse in the Geneva peace talks.
Speaking in London ahead of a conference on the Syrian humanitarian effort, Kerry said he had called Russian Foreign Minister Sergey Lavrov for a "robust" discussion.
In Geneva on Wednesday, UN envoy Staffan de Mistura suspended attempts to begin a dialogue between Bashar al Assad's regime and the Syrian opposition.
Alongside Britain's Foreign Secretary Phillip Hammond, Kerry read out sections of UN Security Council resolution 2254, passed in December last year, calling for an immediate ceasefire.
"Russia has a responsibility, as do all parties, to live up to it," he said.
"So I had a conversation this morning with Foreign Minister Lavrov. We discussed, and we agreed, that we need to discuss how to implement the ceasefire."
Kerry also said that both parties to the conflict - the rebels as well as the regime and its allies - must allow access to besieged areas for humanitarian aid.
"So we had a robust discussion this morning about that. We will be continuing the discussion," Kerry said.
"Foreign Minister Lavrov and I will talk again today or tomorrow as we further this process and find the way forward to be able to implement this resolution fully."
Keeping up ‘momentum’
The Russian Foreign Ministry said the ministers had agreed to do everything possible to make the break in Syrian peace talks "as short as possible."
According to the Moscow read-out, Lavrov also voiced concern about "unacceptable" preliminary conditions being put forward by "some representatives" of the Syrian opposition.
Moscow added that the two top diplomats - who are set to meet for talks in Munich on February 11 - agreed to coordinate possible steps to bring humanitarian aid to affected areas by air.
As part of a contact group known as the International Syria Support Group, Russia and the United States worked together to get the warring parties to the table in Geneva.
But while Washington and its ally Saudi Arabia remain close to the opposition, Moscow has continued to take Assad's side and its aircraft bomb opposition targets daily.
This has contributed to the mood of distrust at the talks, and the United Nations has proved unable to get the sides into serious negotiations on a political settlement.
"Staffan de Mistura has temporarily interrupted the talks in order to try to resolve some of the issues regarding the next steps," Kerry explained.
After a breakfast meeting with a small group of European and Middle Eastern powers, Kerry and Hammond headed into a donors conference to raise money to aid Syrian civilians.
Hammond said the day's focus would be on funding for the refugee crisis and the eventual rebuilding of Syria, but that delegates would not forget about the peace process.
"We're all very keen to keep momentum in that dialogue," the British minister told reporters.
"We recognise that it's difficult for the regime to be at the table, talking to the opposition," he said.
"And it's difficult for the opposition to talk to the regime when their people at home are being killed."