US Secretary of State John Kerry is meeting his Russian counterpart Sergei Lavrov in Geneva for a ceasefire in Syria.

Senior US officials say Kerry would not have agreed to meet Lavrov face-to-face unless real progress was possible.

However, they also warn that they can't guarantee a final agreement will be reached.

Both men will return home later in the day. They have continued to meet on the issue -- most recently on Sunday and Monday on the sidelines of the G20 Summit in China.