Ukrainian President Volodymyr Zelenskiy has returned from a visit to Turkey, bringing home five former commanders of Ukraine's garrison in Mariupol despite a prisoner exchange last year under which the men were meant to remain in Turkey.
Russia immediately denounced the release of the men.
Kremlin spokesman Dmitry Peskov said Turkey had violated the prisoner exchange terms and had failed to inform Russia.
The commanders, lionised as heroes in Ukraine, led last year's defence of the port, the biggest city Russia captured in its invasion.
Thousands of civilians were killed inside Mariupol when Russian forces laid the city to waste during a three-month siege.
Several thousand Ukrainian soldiers, including fighters from the ultranationalist Azov regiment, held out in the Azovstal steelworks.
The Ukrainian defenders, who held out in tunnels and bunkers under a steel plant, were finally ordered by Kyiv officials to surrender in May last year.
Russia freed some of them in September last year in a prisoner swap brokered by Turkish officials, under terms that required the commanders to remain in Turkey until the end of the war.
"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.
"Ukrainian soldiers Denys Prokopenko, Svyatoslav Palamar, Serhiy Volynsky, Oleh Khomenko, Denys Shleha. They will finally be with their relatives," he said on the Telegram messaging app.
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 in the run-up to 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 being allowed to return home.
Turkey's Directorate of Communications did not immediately respond to a request for comment.
Zelenskiy posted a one-minute video showing himself and other officials shaking hands and hugging the smiling commanders before they boarded a Czech plane together.
Many Ukrainians hailed the news on social media.
"Finally! The best news ever. Congratulations to our brothers!" Major Maksym Zhorin, who is fighting in eastern Ukraine, said on the Telegram messaging app.
Russia has repeatedly cited the ultranationalist Azov regiment in claiming that it seeks to "liberate" Ukraine from "fascists".
with DPA
