Negotiators from the United States and Russia sat down on Monday for talks in Saudi Arabia on a partial ceasefire in the Ukraine conflict, hours after a round of negotiations between US and Ukrainian delegations, Russian news reports said.

The state Tass and RIA-Novosti news agencies confirmed that negotiations had begun in the capital Riyadh. The meeting is expected to be followed by another contact between US and Ukrainian teams.

Originally planned to take place simultaneously to enable shuttle diplomacy -- with the US going back and forth between the delegations -- the talks are now taking place one after the other.

The separate meetings are set to discuss details of a pause in long-range attacks from both Russia and Ukraine against energy facilities and civilian infrastructure, as well as a halt on attacks in the Black Sea to ensure safe commercial shipping.

Earlier, US special envoy Steve Witkoff voiced optimism that any agreement struck would pave the way for a "full-on" ceasefire.

"I think you're going to see in Saudi Arabia on Monday some real progress, particularly as it affects a Black Sea ceasefire on ships between both countries. And from that you'll naturally gravitate to a full-on shooting ceasefire," he told Fox News.

But the Kremlin on Sunday downplayed expectations of a rapid resolution.

"We are only at the beginning of this path," Kremlin spokesman Dmitry Peskov told Russian state TV. "There are difficult negotiations ahead."