Updated at 4:15 p.m. on June 22 to clarify that a gas drilling platform, not an oil drilling platform, was struck.
Ukraine has admitted striking a gas drilling platform in the Black Sea, saying Tuesday it was being used by Russian troops as a military installation.
The attack, which took place on Monday, was the first such strike against offshore energy infrastructure in Crimea since Russia invaded Ukraine on Feb. 24.
It targeted a rig with four towers some 70 kilometers off the Russian-annexed Crimean peninsula, which Ukraine said was being used as a military installation.
"On those towers, Russia had organized small garrisons and stored equipment for air defense, radar warfare and reconnaissance," Sergiy Bratchuk of Odessa's regional military administration told an online briefing.
"They were being turned into fortification points that were helping the Russians achieve full control of the northwestern part of the Black Sea," he said, his remarks quoted by Interfax Ukraine.
Sergey Aksyonov, head of Russian-annexed Crimea, had on Monday accused Ukraine of firing on a Black Sea drilling platform off the peninsula, leaving three people injured and another seven missing.
Russian senator for Crimea Olga Kovitidi later said two other platforms had also been hit.
At least one strike was confirmed by Bratchuk, who said such platforms were "legal military targets" for Ukraine.
"This is a small garrison. This is not just a platform for extracting gas," he said in a video posted on his Telegram channel.
Bratchuk also said Ukrainian forces had "dealt a significant blow to the Russian garrison" on Snake Island, a symbolic Black Sea territory taken by Moscow at the start of its Ukraine offensive.
But he acknowledged Russia's Black Sea fleet still dominated the area.
"They are located 80 to 100 kilometers off the coast and the enemy's tactics are missiles, missiles and more missiles," he said.