The detained chief of Ukraine's Russian-held nuclear power plant has been released, the head of the UN nuclear watchdog said on Monday.

"I welcome the release of Ihor Murashov, director general of Ukraine's Zaporizhzhia Nuclear Power Plant; I have received confirmation that Mr Murashov has returned to his family safely," Rafael Grossi, the head of the International Atomic Energy Agency (IAEA) said on Twitter.

Kyiv on Saturday called for the immediate release of Murashov, condemning his "illegal detention".

Zaporizhzhia – Europe's largest nuclear energy facility – has been at the centre of recent tensions after Moscow and Kyiv accused each other of strikes on and near the plant, raising fears of an atomic disaster.

Murashov was detained by a "Russian patrol" on Friday at around 4:00 pm local time (1300 GMT) on his way from the plant to the city of Energodar, the head of Ukraine's nuclear agency Energoatom, Petro Kotin, said in a statement.

He said Murashov's vehicle was stopped, he was taken out of the car and "with his eyes blindfolded, he was driven in an unknown direction".

Ukraine's foreign ministry said in a statement it "condemns in the strongest terms the illegal detention".

Murashov "bears the main and exclusive responsibility for the nuclear and radiation safety" of the Zaporizhzhia plant, according to Kotin.

The nuclear plant is located in Russian-held territory of Ukraine's Zaporizhzhia region, which Russia on Friday annexed together with three other territories in Ukraine: Donetsk, Lugansk and Kherson.

Shelling around the plant has spurred calls from Kyiv and its Western allies to demilitarise areas around nuclear facilities in Ukraine.

A monitoring team from the UN visited Zaporizhzhia in early September, and the IAEA has been pushing Kyiv and Moscow to agree to set up a security zone around the plant to seek to avoid any nuclear accidents.

Grossi said over the weekend that he was expecting to travel to Kyiv and Moscow this week to discuss this. (AFP)