Ukraine's domestic security service, the SBU, allegedly conspired with the Azov Battalion fighters to stage a false-flag attack in Kharkiv, Russia's Ministry of Defence claimed.
According to the Ministry, the Ukrainian ultranationalist militants allegedly rigged a small experimental nuclear reactor at the Institute of Physics and Technology (KIPT) with explosives, RT reported.
"The SBU and the militants of the Azov Battalion are planning to blow up the reactor and accuse the Russian Armed Forces of allegedly launching a missile attack on the experimental nuclear facility," it said in a statement, warning of a risk of radioactive contamination of the area should such a false flag incident take place.
Earlier on Sunday, Ukraine's Internal Affairs Ministry had accused Russian forces of shelling the city, claiming that at least one projectile hit the KIPT, which hosts an experimental neutron source reactor with "37 nuclear fuel elements loaded into its active zone".
The security service also shared several videos of what it claimed to be the Grad multiple rocket launcher shells hitting the perimeter and damaging the side of the building, RT reported.
There were no reports of any radioactive contamination or leaks from the alleged incident in Kharkiv, but Ukraine's Deputy Foreign Minister Emine Dzheppar tweeted that "the destruction of nuclear materials installations and reserves could lead to a major technological and ecological disaster", and accused Russia of "war crimes".
The fresh exchange of accusations comes on the heels of a shootout and fire near Ukraine's Zaporizhzhya nuclear power plant on March 4, which sparked worldwide concern and accusations that Russia was risking a nuclear disaster.
Kiev and Moscow accused each other of provoking the situation.
"Attempts to blame the Russian military for this incident are part of a cynical propaganda campaign," President Vladimir Putin said on Sunday.
The "physical and nuclear safety" of the Zaporizhzhya plant is being maintained by Russian troops in cooperation with Ukrainian security and personnel, Putin said, adding that Russian forces remained in control of the now-defunct Chernobyl nuclear power plant � the site of the biggest nuclear disaster in history.