President Volodymyr Zelenskyy has returned home, from a visit to Turkey, with five commanders of Ukraine’s former garrison in Mariupol.
Russia has said this move violated the terms of a prisoner exchange deal engineered last year.
Kremlin spokesman Dmitry Peskov said Ankara had promised under the exchange agreement to keep the men in Turkey and complained that Moscow had not been informed of the move.
The five commanders, lionised as heroes in Ukraine, led last year’s defence of the southern port of Mariupol, the biggest city Russia has captured in its invasion.