Each side is due to meet on Friday with Marco Rubio, the US Secretary of State, and Turkey's foreign minister, Hakan Fidan, but still without any broader meeting planned.

Putin's absence punctured hopes of a breakthrough in peace efforts that were given a push in recent months by the Trump administration and Western European leaders.

The president said he's optimistic about Thursday's expected talks between Vladimir Putin and Volodymyr Zelensky in Istanbul.