Crimean Parliament has approved a bill seizing assets of Ukrainian oil and gas enterprises operating in the region.
Resources owned by Chernomorneftegaz and Ukrtransgaz, entities dealing with extraction and transportation of fuel, have thus passed into the hands of local authorities, Russian agency RIA Novosti has reported.
The move was justified with the needs of energy and environmental security.
The two companies will now be called "Crimean Republican Enterprises".
They will be controlled by Crimea's newly-created Ministry of Fuel and Energy, which also seems to control oil and gas ports across the peninsula.
Russian website RBC Daily quotes Crimea's Parliament Speaker Vladimir Konstantinov as saying that oil and gas deposits in both Black and Azov Sea are already being guarded by Crimean authorities as they are now owned by the republic.
He also claimed, however, that issues regarding oil and gas extraction in Crimea must be dealt with by Russia.
"It is necessary for Russia, Gazprom to take charge of oil and gas extraction. This is not our business," Konstantinov has said.
Over the past few weeks, Crimea has taken steps toward splitting from Ukraine well before March 16's referendum, such as setting up its own institutions.
Earlier on Monday, short after results of the poll were announced and it sent a formal request to join Russia, the government declared that Crimea was preparing to adopt Russian currency, the rouble, as well as to use Moscow Standard Time, which is currently two hours ahead of Kiev.