Ukraine's Defense Ministry said late on Friday it had "temporarily" lost access to the Sea of Azov after Russian forces tightened their grip on the main sea port of Mariupol.
"The occupiers succeeded in part in the Donetsk region of operations, temporarily depriving Ukraine of access to the Sea of Azov," the ministry said in a statement.
In its statement, the ministry did not specify whether the Ukrainian forces had regained access to the sea.
Russia said on Friday that its forces were "tightening the noose" around Mariupol, where about 80% of its homes were damaged, and about 1,000 people may still be trapped in makeshift shelters.
Mariupol - with its strategic location on the coast of the Sea of Azov - has been a target since the start of the war on February 24, when Russian President Vladimir Putin launched what he described as a "special military operation".
The city lies on the road between Russia's annexed Crimea in the west and Donetsk region in the east, which is partly controlled by pro-Russian separatists.