The recapture of the so-called Boyko Towers platforms provides an energy source and takes back an asset that Russia seized in 2015 and used to launch helicopters, Ukraine's Ministry of Defence said.
"Russia has been deprived of the ability to fully control the waters of the Black Sea, and this makes Ukraine many steps closer to regaining Crimea," the Main Intelligence Directorate said.
Ukrainian President Volodymyr Zelenskiy has vowed to do all he can to bring back Crimea, which Russia annexed from Ukraine in 2014, and has urged international allies to support the effort.
Ukraine's reported battlefront gains, which could not be independently confirmed, came as North Korean leader Kim Jong-un was reportedly on a train headed for Russia for a meeting with Russian President Vladimir Putin.
The meeting announced by the two leaders' countries might include discussions of North Korea providing arms to restock Russia's dwindling arsenal.
US officials released intelligence last week indicating that North Korea and Russia were arranging a Putin-Kim meeting for sometime this month as they expand co-operation in the face of deepening confrontations with the United States.
In other developments, German Foreign Minister Annalena Baerbock made an unannounced visit to Kyiv, Ukraine's capital.
She promised support for Ukraine's path toward European Union membership while calling for additional reforms in the country.
"With enormous courage and determination, Ukraine is also defending the freedom of all of us," Baerbock said in a statement released by her ministry.
"In the same way that Ukraine stands up for us, it can also count on us."
Baerbock also pledged continued military, economic and humanitarian support for the country and said the 22 billion euros ($A37 billion) provided so far now made Germany second to the US in terms of total support.
Baerbock said that while Ukraine had already made good progress reforming the judiciary and the media, it still had "some way to go" in combating corruption.
In fighting, Ukrainian forces liberated part of the Donetsk province town of Optyne and advanced on the towns of Klishchiivka and Andriivka south of Bakhmut, Deputy Defence Minister Hanna Malyar said.
Combat has persisted on the outskirts of Bakhmut since Ukrainian troops pulled out of the city in May.
Ukraine is trying to gain the high ground in Klishchiivka, to establish artillery control over Bakhmut.
In southern Ukraine's Zaporizhzhia province, Ukraine's primary counteroffensive forces were inching closer to overcoming Russian fortifications and dense minefields to take Tokmak, a critical logistics hub for Russian forces and a vital railway junction, Malyar said.
Ukrainian forces liberated Robotyne, a town in the same province, last month.
Russian forces also attacked the Dnipropetrovsk province city of Kryvyi Rih, Zelenskiy's birthplace, with drones overnight, Ukrainian authorities said.
The retaking of the Black Sea platforms follows the United Kingdom Ministry of Defence reporting naval and air force skirmishes at sea two weeks ago.
Ukraine has struck several Russian-controlled platforms in fighting during the war, and troops from both countries have occupied them periodically, the UK said in a military update on the war.
Along with drilling, the platforms can be used to land helicopters as deployment bases and to position long-range missile systems.
Pro-Russia occupation authorities seized the platforms operated by the Chernomorneftegaz company following the annexation of Crimea, which most of the world regarded as illegal.
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