The United Nations nuclear watchdog has lost touch with its remote systems that monitor nuclear material at the Zaporizhzhia nuclear power plant in Ukraine, it said on Wednesday.

Zaporizhzhia, Europe's largest nuclear power plant, has been in Russian hands since last week, when a blaze broke out in a building at the site after clashes between Russian and Ukrainian forces.

The International Atomic Energy Agency (IAEA) announcement came a day after it said the same thing had happened at the radioactive waste facilities at Chernobyl, near the defunct power plant that was the site of the world's worst nuclear disaster in 1986.

Both sites are under Russian forces' control but are being operated by Ukrainian staff in conditions that the IAEA says endanger the safety of the facilities.

IAEA chief Rafael Grossi is "concerned about the sudden interruption of such data flows to the IAEA's Vienna headquarters from the two sites, where large amounts of nuclear material are present in the form of spent or fresh nuclear fuel and other types of nuclear material," the IAEA said in a statement

It was not clear what had caused the disruption, the IAEA said, adding that transmissions from other nuclear facilities in Ukraine, including its three other operational nuclear power plants, were continuing. The status of the equipment that had stopped transmitting was "uncertain", it said.

The agency said Zaporizhzhia's operator had informed it that two of its four external high-voltage power lines had been damaged so only two were now available. It only needs one, and there is a fifth on standby, plus backup diesel generators.

One unit's transformer was also undergoing emergency repair after damage to its cooling system was discovered "following the events" of March 4, the date of the fire and clashes, it added.

"These recent developments added to the IAEA's growing concerns about the safety, security and safeguards impact of the conflict in Ukraine on the country's nuclear facilities," the IAEA said.

Grossi has called for a trilateral meeting with Ukraine and Russia to ensure the safety of Ukraine's nuclear facilities. He said on Wednesday he would travel to the Turkish city of Antalya on Thursday, where the Russian and Ukrainian foreign ministers are due to meet.

Earlier on Wednesday Ukraine said power to Chernobyl had been cut but the IAEA said the spent fuel there could still be cooled safely.