The UN nuclear chief said Thursday he is sending inspectors to two locations in Ukraine where Russia alleged that activities related to the possible production of dirty bombs was taking place.

Rafael Grossi said in a statement that inspectors from the International Atomic Energy Agency would be travelling this week to the two sites, which are under IAEA safeguards, following a written request from the Ukrainian government.

"The purpose of this week's safeguards visits is to detect any possible undeclared nuclear activities and materials related to the development of dirty bombs," the statement said.

The IAEA inspected one of the two locations a month ago and no undeclared nuclear activities or materials were found there.

The statement did not identify the sites, but Russia's UN ambassador alleged in a letter to Security Council members this week that Ukraine's Institute for Nuclear Research of the National Academy of Sciences in Kyiv and Vostochniy Mining and Processing Plant have received direct orders from (President Volodymyr) Zelenskyy's regime to develop such a dirty bomb.

The envoy, Vassily Nebenzia, said that information was from Russia's Ministry of Defence.

He said that the works are at their concluding stage on a dirty bomb, which uses explosives to scatter radioactive waste in an effort to sow terror.

Grossi was at UN headquarters in New York briefing council members behind closed doors Thursday.

Russian Defense Minister Sergei Shoigu made an unsubstantiated allegation that Ukraine was preparing to launch a so-called dirty bomb over the weekend in calls to his British, French, Turkish and U.S. counterparts.

Britain, France and the United States rejected the claim out of hand, calling it transparently false.

Russian President Vladimir Putin repeated the allegation Wednesday.

Ukraine dismissed Moscow's claim as an attempt to distract attention from the Kremlin's own alleged plans to detonate a dirty bomb.

Energoatom, the Ukrainian state enterprise that operates the country's four nuclear power plants, said Russian troops have carried out secret construction work over the last week at the occupied Zaporizhzhia Nuclear Power Plant.

"Russian officers controlling the area won't give access to Ukrainian staff running the plant or monitors from the U.N.'s atomic energy watchdog that would allow them to see what the Russians are doing," Energoatom said in a statement Tuesday.