US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov failed to make progress on a Syria truce in talks in Germany early Thursday, an American official said.
Kerry and Lavrov met twice Thursday on the margins of a foreign ministers' meeting of the 57-member Organisation for Security and Cooperation in Europe, said a US official.
Their meetings lasted 10 minutes each and "no progress, no conclusion were reached on Aleppo", the devastated north Syrian city where Russia-backed regime troops are battling rebel forces, the official said.
However, leaving the talks with Lavrov in the northern German city of Hamburg, Kerry told Russian journalists that he remained hopeful and would continue to work for a solution.
Moscow launched an air war in support of Assad's forces last year, while Washington has supported rebel forces battling the regime.
On Wednesday the United States, Britain and France led a joint call for an immediate ceasefire to allow aid to reach Aleppo, in an appeal backed by Canada, Germany and Italy.
Kerry said at a meeting of NATO foreign ministers in Brussels on Tuesday that he would work for a relaunch of peace talks between the Syrian regime of President Bashar al-Assad and the opposition, with the help of Russia.
"Russia says Assad is ready to come to the table ... and I am in favour of putting that to the test," Kerry said in Brussels.
Lavrov said in Hamburg on Wednesday, before a meeting with Kerry, that Russia backed a US proposal made on December 2 involving a rebel withdrawal from eastern Aleppo, which Moscow has accused Washington of since backtracking on.
Kerry has denied any change of plans and Washington itself has accused Moscow of stalling after Russia and China blocked a UN Security Council resolution on Monday calling for a seven-day ceasefire.
Kerry will on Saturday take part in a Paris meeting on Syria organised by his French, German and Qatari counterparts, the State Department said.