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

He warned, however, that any future damage to equipment in the core could pose a danger.

"The facility, in working condition, doesn't present any danger whatsoever," said Mykola Shulga, director general of the National Science Centre Kharkiv Institute of Physics and Technology.

"However, if there is physical damage, a nuclear fuel leak is possible, radioactive elements (could escape) outside," he told Reuters, speaking inside the facility.

"This obviously would be a huge, huge problem for the environment. In other words, what would happen would be comparable to a similar situation at any nuclear power station."

While the inner areas of the institute were unscathed, some of the outer walls of the building were damaged by shrapnel and windows were blown in.

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

Zaporizhzhia, Europe's largest nuclear power plant, is now in Russian hands. During fighting with Ukrainian forces in the area, a blaze broke out in a building at the site.

The International Atomic Energy Agency (IAEA), the U.N. nuclear watchdog, said something similar 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.

Shulga dismissed recent reports in Russian media that Ukraine was developing a "dirty bomb" nuclear weapon. The source-based reports did not cite evidence.

"It is impossible to make nuclear weapons out of those nuclear fuel cells," he said.

"The fake news that has been circulating in media recently about our institute working to produce nuclear weapons is absolutely not true. What's more, everything that is being worked on at the institute is fully controlled by the IAEA."

Russia calls its actions in Ukraine a "special military operation" to disarm and "de-Nazify" the country. It denies targeting civilians.

Ukraine and its allies accuse Moscow of an unprovoked invasion that has caused a humanitarian catastrophe in which hundreds of civilians have been killed and millions displaced.

Kharkiv, Ukraine's second largest city, has seen some of the worst bombing since the conflict began on Feb. 24.

(Writing by Mike Collett-White, Editing by Rosalba O'Brien)