A senior U.S. negotiator on Monday welcomed weekend talks in Moscow and Kyiv, saying the discussions could pave the way for a new round of three-way negotiations aimed at ending the Russia-Ukraine war.

"Substantive progress was made, including movement on scheduling additional trilateral talks," U.S. Special Envoy Steve Witkoff wrote on X.

Witkoff, a business ally of President Donald Trump, traveled to Russia and Ukraine over the weekend alongside negotiating partner Jared Kushner, who is Trump's son-in-law.

Sunday's appearance in Kyiv was their first visit to the Ukrainian capital.

A senior Ukrainian official told AFP on Sunday that the two envoys had brought new and "more effective" proposals on ending the war, but Ukrainian President Volodymyr Zelensky warned that fighting appeared likely to continue into the winter.

On Monday, the Kremlin did not rule out three-way talks with the United States and Ukraine aimed at ending the conflict.

Negotiators from Moscow and Kyiv last met for US-organized talks in Geneva in February, but did not reach an agreement on territorial issues.

Diplomatic efforts led by Washington have slowed in recent months as the Trump administration shifted focus toward the war with Iran.

Russia's full-scale invasion of Ukraine, launched in February 2022, has resulted in the deaths of at least 15,000 civilians, according to the United Nations, although it says the actual number is likely considerably higher.