Ukrainian President Volodymyr Zelenskyy says 144 Ukrainian fighters have been released from Russian captivity as part of a prisoner swap.
Ukrainian military intelligence said it is the largest prisoner swap since the start of the war.
Of those released, 95 were involved in defending the Azovstal steel plant in Ukraine's devastated southern city of Mariupol before Russian forces captured it weeks ago.
Denis Pushilin, the separatist leader in Donetsk, said an equal number of soldiers was released by both sides.
Zelenskyy also said that five people were killed by a Russian missile strike on Mykolaiv.
"Just a Russian strike on Mykolaiv today, with 10 missiles hitting only civilian sites, proves alone for everyone around the world that the pressure on Russia isn't sufficient," he added.
Zelenskyy also said Ukraine would cut ties with Syria over its recognition of Ukraine's eastern Luhansk and Donetsk regions as "independent and sovereign".
Syria is a strong ally of Russia, which joined Syria's conflict in September 2015 helping tip the balance of power in favour of Assad.