US Secretary of State John Kerry warned on Wednesday that Russia's aerial bombardment of Syrian opposition targets could derail efforts to restart talks to end the country's brutal civil war.
Kerry and his Russian counterpart Foreign Minister Sergei Lavrov will meet in Munich on Thursday to host a meeting of a 17-nation contact group, designed to get the talks back on track.
But, US's frustration with Russia's support of Bashar al-Assad's regime is growing, amid fears that the Opposition will refuse to join UN-led peace talks while their cities are under fire.
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"And, we have called on Russia again to join in the effort to bring about an immediate ceasefire and to bring about full humanitarian access," he said.
"That is what this meeting will be about and this meeting will tell a lot about the road ahead," he added, in remarks at the State Department ahead of talks with Egyptian Foreign Minister Sameh Shoukry.
Russia is a signatory to UN Security Council resolution 2254, passed in December, which orders all warring parties in Syria to respect an immediate ceasefire and allow humanitarian agencies to take food and relief supplies to besieged civilians.
But, with UN-led peace talks in Geneva stalled, Russian jets have continued to bomb Assad's enemies, most recently in an onslaught that has seen regime forces advance to all but encircle the city of Aleppo.
Last week, UN envoy for Syria Staffan de Mistura suspended talks amid opposition protests about the bombing, and Thursday's meeting of the 17-nation International Syria Support Group aims to pressure the sides to return to the table.