Russia has accused Turkey of breaching international agreements by releasing detained commanders of a Ukrainian army unit that defended a steel works in the city of Mariupol.
Ukrainian President Volodymyr Zelenskyy returned to Ukraine from Turkey and brought back with him the freed five commanders of the former garrison in Mariupol.
Kremlin spokesman Dmitry Peskov said the fighters were supposed to remain in Turkey until the end of the war under the terms of a prisoner exchange.
The Kremlin was not informed of their release, Mr Peskov was quoted by RIA news agency as saying.
The commanders were hailed as heroes in Ukraine for their defence of the port city, which was subjected to a three-month siege by Russian forces.
Mr Zelenskyy announced: “We are returning home from Turkey and bringing our heroes home.”
He also noted the names of the released commanders, who will be reunited with their families.
Black Sea grain deal and prisoner exchanges
Turkish President Recep Tayyip Erdogan announced on Saturday that he was pressing Russia to extend a Black Sea grain deal by at least three months, and a visit from President Vladimir Putin was scheduled for August.
Mr Erdogan and Mr Zelenskyy held a joint news conference after discussing the fate of the arrangement brokered last year by Turkey and the UN to ensure the safe export of grain from Ukrainian ports through the Black Sea amid the war.
Mr Erdogan said work was under way to extend the grain deal and hoped that it would be renewed at least once every three months.
The question of prisoner exchanges was also a key point during Mr Erdogan's talks with Mr Putin.
Mr Zelenskyy said it was the first thing on their agenda and expressed hope for a favourable result.
He also indicated that they had detailed discussions about the return of all captives, including children taken to Russia and other groups.
Mr Erdogan also said that the issue could come up in his contacts with Mr Putin before his visit.
Turkey's diplomatic balance act
Turkey, a Nato member, has maintained cordial relations with both Russia and Ukraine throughout the 16-month war and has played a significant role in brokering prisoner exchanges.
While Turkey has not imposed economic sanctions on Russia like its Western allies, it has supplied arms to Ukraine and called for respect of its sovereignty.
Despite the ongoing tensions, the Kremlin has acknowledged Mr Erdogan's efforts in mediating a resolution of the conflict in Ukraine.
