Russia has proposed a March 1 ceasefire in Syria, U.S. officials say, but Washington believes Moscow is giving itself and the Syrian government three weeks to try to crush moderate rebel groups.
The United States has countered with demands for the fighting to stop immediately, the officials said Wednesday. Peace talks are supposed to resume by Feb. 25.
In the meantime, an official statement came from Russia as the Foreign Minister Sergei Lavrov said on Thursday that Russia has made a "quite specific" proposal for a ceasefire in Syria and is awaiting a US response.
"We made propositions for a ceasefire that are quite specific," he said as he sat down for talks with his US counterpart John Kerry in Munich.
"We will wait for the American response before we take it to the (International Syria Support Group)."
"We're going to have a serious conversation about all aspects about what's happening in Syria. Obviously, at some point in time, we want to make progress on the issues of humanitarian access and ceasefire," Kerry said.
Kerry and Lavrov will host foreign ministers from the 17-nation Syria contact group later Thursday, for a meeting billed as a moment of truth for the floundering peace process.
The Syrian war has killed more than a quarter-million people, created Europe's biggest refugee crisis since World War II and allowed the Daesh to carve out its own territory across parts of Syria and neighboring Iraq.
Russia says it is supporting Assad's government as part of a counterterrorism campaign. But the West says the majority of its strikes are targeting moderate groups that are opposed to Assad and the Daesh as well as civilians.
The most recent Russian-backed offensive, near Aleppo, prompted opposition groups to walk out of peace talks last month in Geneva, while forcing tens of thousands of civilians to flee toward the Turkish border.
The U.S. officials weren't authorized to speak publicly about private diplomatic discussions in the run-up to the Munich conference and demanded anonymity. One said the U.S. can't accept Russia's offer because opposition forces could suffer irreversible losses in northern and southern Syria before the ceasefire ever takes hold.
The officials said the U.S. counterproposal is simple: A ceasefire that is effective immediately and is accompanied by full humanitarian access to Syria's besieged civilian centers.