On Saturday he met Russia's Foreign Minister, Sergei Lavrov, along with representatives from other countries with stakes in the five-year conflict.
Nothing concrete came from that but both the US and Russia spoke of "ideas" emerging.
A ceasefire brokered last month collapsed after just a few days.
Since then Syrian forces, backed by Russia, have been bombing the city of Aleppo - the most high-profile battleground in the country's civil war - in what Washington has called a war crime.
