"The US continues to push for an immediate ceasefire. We are continuing to work through various ways to achieve one as soon as possible," the diplomat said.
The comment came in response to reports that Russia has proposed a ceasefire that would not begin until March 1, in what opponents see as a bid to buy time for a Syrian government offensive in and around Aleppo.
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Government forces, backed by Russian bombers and Iranian fighters, have advanced on the embattled rebel-held city in fighting that the United Nation says has displaced 51,000 civilians.
Washington wants a ceasefire and humanitarian access to besieged rebel cities but has threatened an unspecified "Plan B" if talks fail, as tension mounts with Moscow over its air campaign.
Russian Foreign Minister Sergei Lavrov said Moscow has made a "quite specific" ceasefire proposal but provided no other details as he sat down in Munich for talks with US Secretary of State John Kerry.
"We will wait for the American response before we take it to the (International Syria Support Group)," he said, referring to the 17-nation contact group meeting in Munich on Syria.