US Secretary of State John Kerry flew in to Moscow on Tuesday to meet Russia’s President Vladimir Putin and urge him to keep the fledgling Syrian peace process on track.

US Secretary of State John Kerry flew in to Moscow on Tuesday to meet Russia's President Vladimir Putin and urge him to keep the fledgling Syrian peace process on track.

Washington is relying on the Kremlin to convince Syrian Bashar al-Assad to participate in talks with the opposition factions and armed groups sitting together in Riyadh.

Kerry and the UN envoy for Syria, Staffan de Mistura, want to hold the next round of International Syrian Support Group talks on Friday in New York, but Moscow has so far refused to confirm the date.

Moscow said on Monday that Friday's talks should not take place until both the rebel representation and the terror blacklist are agreed upon by all parties.

Kerry is in near constant touch with his Russian counterpart Sergey Lavrov, but knows that the road to peace talks runs through the Kremlin.

"Obviously, he has a chance to meet with Lavrov all over the world," a senior US official said.

"President Putin is the decision-maker in Russia. It's important to have a chance to talk to him directly."