President Volodymyr Zelenskiy brought home from Turkey on Saturday five former commanders of Ukraine’s garrison in Mariupol, a highly symbolic achievement that Russia said violated a prisoner exchange deal engineered last year.

Russia immediately denounced the release. Kremlin spokesman Dmitry Peskov said Ankara had promised under the exchange agreement to keep the men in Turkey and complained Moscow had not been informed.

In honour of the 500th day of the war, Zelenskiy also visited Snake Island, a Black Sea outcrop which Russian forces seized on the day of the invasion and later abandoned.

The five commanders have been lionised in Ukraine after leading a fierce three-month defence of Mariupol from the Azovstal steel plant last year, the biggest city Russia has captured.

“We are returning home from Turkey and bringing our heroes home,” said Zelenskiy, who met Turkish President Tayyip Erdogan for talks in Istanbul on Friday.

Thousands of civilians were killed in Mariupol when Russian forces laid the city to waste in the first months of the war. The Ukrainian defenders held out in tunnels and bunkers under the Azovstal plant, until finally ordered by Kyiv to surrender in May last year.

Moscow freed some of them in September in a prisoner swap brokered by Ankara, under terms that required the commanders to remain in Turkey until the end of the war.

Peskov told Russia’s RIA news agency: “No one informed us about this. According to the agreements, these ringleaders were to remain on the territory of Turkey until the end of the conflict.”

Peskov said the release was a result of heavy pressure from Turkey’s NATO allies ahead of next week’s summit of the military alliance at which Ukraine hopes to receive a positive sign about its future membership.

In his remarks, Zelenskiy gave no explanation for why the commanders were allowed to return home now. Turkey’s Directorate of Communications did not respond to a request for comment.