John Kerry and his Russian counterpart Foreign Minister Sergei Lavrov will host a meeting of a 17-nation contact group.
US Secretary of State John Kerry warned Tuesday that Russia's aerial bombardment of Syrian opposition targets could derail efforts to re-start talks to end the country's brutal civil war.
Kerry and his Russian counterpart Foreign Minister Sergei Lavrov are due in Munich on Thursday to host a meeting of a 17-nation contact group designed to get the talks back on track.
But US frustration with Russia's bombing in 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.
"There is no question, and I have said this before publicly, that Russia's activities in Aleppo and in the region right now are making it much more difficult to be able to come to the table and to be able to have a serious conversation," Kerry said.
"And we have called on Russia and we call 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.