The United States and Russia on Friday grappled over plans to halt the fighting in Syria, as resurgent Moscow-backed regime forces tightened the noose around the beleaguered city of Aleppo.
In Geneva, Secretary of State John Kerry was once more locked in talks with his Russian counterpart Sergei Lavrov, but US officials warned that negotiations could not go on forever without a breakthrough.
Washington wants Moscow to help clinch a ceasefire, get humanitarian aid to civilians and -- eventually -- set the stage for political talks to end a five-year war that has killed more than 290,000.
The two powers back opposite sides in the civil war, with Moscow supporting Bashar al-Assad's Syrian regime and Washington behind a coalition of rebel groups it regards as moderate.
Senior officials travelling with Kerry said he would not have flown out once again to new face-to-face talks with Lavrov unless he thought there was a chance of progress.
A US official described the talks as "crisp and businesslike", focused on specific technical details of how the ceasefire would be observed.
In a break in proceedings, the US delegation was to update Washington on progress. But officials warned there was no guarantee of a final agreement before both men return home later Friday, just four days after the pair met in China and failed to narrow their differences.