Setting out Kyiv's priorities, the head of Ukraine's delegation said peace was only possible if Russia agreed to a 30-day ceasefire, and an exchange of all prisoners of war, among some other things.

Russia says it wants to end the conflict by diplomatic means and is ready to discuss a ceasefire, but has also raised a list of questions and concerns, saying Ukraine could use the pause to rest its forces, mobilise extra troops and acquire more western weapons.

But a major breakthrough is unlikely, said US Secretary of State Marco Rubio, who is also in Istanbul, where a flurry of separate meetings took place earlier on Friday.

Previously, President Trump also said there would be no movement without a meeting between himself and Russia's President Vladimir Putin, further denting expectations. Trump, winding up a Middle East tour and heading back to Washington, said on Friday he would meet the Russian leader "as soon as we can set it up".