The United States on Friday accused Russia of violating nuclear safety principles in Ukraine and demanded its invading forces stop firing on nuclear power plants, but added that there were no signs detected yet of any radiological release.

US Energy Secretary Jennifer Granholm said in a Twitter post that radiation monitors in much of Ukraine were still functioning although the United States was concerned by the lack of data from safeguards monitors at Chernobyl or Zaporizhzhia, which is Europe's largest nuclear power plant.

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

"We remain concerned about Russia’s reckless actions and violations of nuclear safety principles," Granholm said.

"We are monitoring reports of damage to a research facility in Kharkiv. Near-term safety risk is low, but the continued Russian firing on nuclear facilities must cease," she said.

After fighting and apparent shelling around the Zaporizhzhia plant a week ago, the Russian military accused Ukrainian forces of a "provocation".

Granholm said the US could not confirm reports that power has been restored to Chernobyl. Russia's energy ministry said on Thursday that Belarusian specialists had restored electricity supply to the Chernobyl nuclear power plant.

Ukraine and its allies are concerned about the risk Russia's invasion poses to nuclear facilities across the country, including power plants and research centers.

The head of a nuclear research facility in the northeastern Ukrainian city of Kharkiv said the grounds of the institute had been struck by Russian shells during recent fighting, but the core housing nuclear fuel remains intact.

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(Edited by : Thomas Abraham)