Russia's defence ministry said a Russian warplane struck and killed an unspecified number of Ukrainian troops on Snake Island in the Black Sea on Thursday after they landed there to raise the Ukrainian flag.
"At about 5 am, several Ukrainian servicemen landed on the island from a motor boat and took pictures with the flag. An aircraft of the Russian Aerospace Forces immediately launched a strike with high-precision missiles on Snake Island, as a result of which part of the Ukrainian military personnel was destroyed," Russian defence ministry spokesman Igor Konashenkov said.
Andriy Yermak, the Ukrainian President's chief of staff, earlier posted a video on Telegram of three soldiers raising a large Ukrainian flag on the island, from which Russian forces withdrew on June 30 after coming under heavy bombardment from Ukrainian artillery.
The tiny island, captured by Russia from Ukraine at the start of the war in February, is strategically important because of its proximity to sea lanes close to Ukraine's port of Odesa.
Russia said it had pulled out from Snake Island last week as a "gesture of goodwill" to show it was not obstructing United Nations attempts to open a humanitarian corridor allowing grain to be shipped from Ukraine.
Ukraine said it had driven the Russian forces out after an artillery and missile assault.