Russia forces have fired rockets at physics institute in Ukraine’s eastern city of Kharkiv that contains nuclear materials and a research reactor, Ukraine’s national security services have said.

The incident represents the third time a Ukrainian nuclear facility has fallen within the crosshairs of Russian forces since the invasion began.

On Monday, Russian air attacks on the city of Mykolaiv raised fears that yet another nuclear facility – the South Ukraine nuclear power plant – could soon fall under Russian control.

The strike on the Kharkiv Institute of Physics and Technology nuclear facility came on Sunday, the eleventh day of the Russian invasion, and the security service said it risks a “large-scale ecological disaster.” The International Atomic Energy Agency reported on Monday that the facility had been entirely destroyed, but later walked that claim back.

Th institute houses a nuclear research facility called Neutron Source, in the active zone of which 37 nuclear fuel cells are said to be loaded.

Ukraine claimed that Moscow’s forces are firing missiles from truck-mounted “Grad” launchers, which do not have precise targeting, raising concerns that one would go astray.

Footage published by Ukraine’s ministry of internal affairs on Facebook showed blasts hitting a building – supposedly at the Kharkiv institute – but there were no immediate reports of any damage to the nuclear materials inside.

On February 25^th, Russian forces took control of Chernobyl, the site of the world’s worst nuclear accident, which still houses 20,000 spent nuclear fuel assemblies and an extensive infrastructure for dismantling the plant’s destroyed Number 4 reactor.

Then on Friday, the Zaporozhe nuclear power plant – the largest in Europe – fell to Russian forces after shelling started a fire at a laboratory on the plant’s territory. In both circumstances, the IAEA says the facilities are still being run by their Ukrainian staff.

But it remains unclear whether personnel fresh personnel are being rotated in on shifts to continue operations.

Over the weekend, the IAEA reported it was alarmed by developments at the Zaporozhe plant, where Ukrainian authorities says staff are now operating under orders from Russian commanders.

Russian forces at the plant have also switched off some mobile networks and the Internet “so that reliable information from the site cannot be obtained through the normal channels of communication,” the IAEA statement said. Mobile phone communication was still possible, but with poor quality, the IAEA added.

“I’m extremely concerned about these developments that were reported to me today,” IAEA General Director Raphael Grossi said. “In order to be able to operate the plant safely and securely, management and staff must be allowed to carry out their vital duties in stable conditions without undue external interference or pressure.”

Ukraine is home to 15 nuclear reactors spread across four nuclear power plants, which supply more than half the country’s electricity. It also houses Europe’s second largest quantity of radioactive waste, and two storage sites have also been hit by shelling.