US Secretary of State John Kerry is in Geneva in an attempt to bolster a fragile, partial ceasefire in Syria.
Kerry is due to hold urgent talks on the situation with the UN envoy to Syria, Staffan de Mistura, and the Saudi and Jordanian foreign ministers.
A top priority, he said, was ending the violence in the northern city of Aleppo.
More than 250 civilians have been killed in Aleppo in the last 10 days.
A Russian defence official said earlier that negotiations were taking place to establish a ceasefire there.