U.S. Secretary of State John Kerry says Russia's involvement in Syria is making it difficult to achieve a negotiated end to the nearly five-year war.
Syrian government forces backed by Russian air strikes have laid siege to several rebel strongholds, a move that prompted the suspension of UN-mediated peace talks in Geneva last week.
"Russia's activities from Aleppo and in the region are making it much more difficult to be able to come to the table and be able to have a serious conversation," Kerry said on February 9.
Kerry is expected to meet his counterpart Russian Foreign Minister Sergei Lavrov in Munich on February 10.
Lavrov said Moscow has presented Washington with new proposals for ending Syria's war.
Lavrov said in an interview with the Russian daily Moskovsky Komsomolets published on February 9 that the plan is specific and simple, but does not provide details.
