US Secretary of State John Kerry warned on Wednesday that Russia's backing for President Bashar al-Assad risked further escalating the Syria conflict.

"I made clear that Russia's continued support for Assad risks escalating the conflict and undermining our shared goal of fighting extremism," Kerry told a press conference a day after talks with his Russian counterpart Sergei Lavrov.

Kerry said he had urged Moscow to play a "constructive role" in the search for a political solution in Syria and the fight against the Islamic State group.