The head of Ukraine’s state nuclear company urged the U.N. nuclear watchdog on Thursday to help ensure Russian nuclear officials do not interfere in the operation of nuclear power plants occupied by Russian forces.

Since invading Ukraine on Feb. 24, Russian troops have occupied Europe’s largest nuclear power plant at Zaporizhzhia and the now defunct plant at Chernobyl, scene of the world’s worst nuclear accident in 1986.

Energoatom CEO Petro Kotin said earlier this month that Russia’s state nuclear company Rosatom had sent officials to the Zaporizhzhia plant to try to take control of it. Rosatom said a “limited number” of its officials were there but denied they had taken operational control.

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Calling on the International Atomic Energy Agency (IAEA) to prevent what Ukraine regards as interference by Russian nuclear officials, Kotin said: “(The IAEA) can influence this and they must influence this, and this question will be discussed.”

Kotin was speaking following a meeting in Ukraine on Wednesday with visiting IAEA chief Rafael Grossi.

The IAEA has agreed to establish online monitoring missions to Chernobyl and Zaporizhzhia, Kostin said.

Grossi arrived in Ukraine on Tuesday to start providing assistance, including experts and equipment aimed at keeping nuclear facilities there safe.

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Since Russia’s invasion, Grossi has called on both countries to urgently agree a framework to ensure nuclear facilities are safe and secure.

Ukraine has repeatedly expressed safety concerns about Chernobyl and demanded Russian forces occupying the plant pull out of the area.

The Russian military said after capturing the plant that radiation was within normal levels and their actions prevented possible “nuclear provocations” by Ukrainian nationalists.Russia has denied that its forces have put nuclear facilities inside Ukraine at risk.