UNITED NATIONS — The U.N. 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 and expects them to reach a conclusion “in days – very fast.”

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

Russia’s U.N. 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) Zelensky’s regime to develop such a dirty bomb.”

The envoy, Vassily Nebenzia, said that information was from Russia’s Ministry of Defense. He said the ministry reported that work on a dirty bomb, which uses explosives to scatter radioactive waste in an effort to sow terror, is “at their concluding stage.”

Grossi said: “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 IAEA inspected the nuclear research institute in Kyiv a month ago “and no undeclared nuclear activities or materials were found there,” he said.

But Grossi said the inspectors are going to revisit the facility with a different aim.

Normally inspectors look for nuclear material such as enriched uranium, plutonium and thorium, he said, but in this case “there is mention of certain isotopes, cesium and strontium. So, we are going to be performing a different kind of work to determine whether the fuel there has been reprocessed in some way to extract this.”

Grossi came to U.N. headquarters in New York to brief Security Council members behind closed doors on nuclear issues related to Ukraine. The IAEA earlier issued a statement from him and he spoke to reporters after the council meeting.

Russia’s Nebenzia said he told Grossi that “he should be vigilant” because the two sites are not the only places where dirty bombs can be produced.

Grossi said he remains “extremely concerned” about the possibility of a nuclear accident.

He said that in the coming weeks the IAEA is going to be deploying more experts at other nuclear power plants in Ukraine – Rivni, Khmelnytskyi South Ukraine and Chernobyl. The latter was the scene of the world’s worst nuclear disaster in 1986, and it was occupied by Russian forces soon after their Feb. 24 invasion of Ukraine, though they left at the beginning of April.

The nuclear issue was heightened by Russian Defense Minister Sergei Shoigu’s unsubstantiated allegation that Ukraine was preparing to launch a dirty bomb in weekend 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 unsubstantiated claim on 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.

Grossi said in the statement that nuclear safety and security at Zaporizhzhia “remains precarious” and engineers have been working “to stabilize the plant’s fragile external power supplies following repeated outages earlier this month that forced it to temporarily rely on its emergency diesel generators for electricity.”

The IAEA chief said he made clear that he considers Zaporizhzhia “a Ukrainian plant” and expressed concern at possibe confusion about the chain of command following Russia’s announcement that it has taken control of the facility.

This “could negatively affect nuclear safety and security,” Grossi’s statement said, pointing to plans by senior Ukrainian operating staff to restart a reactor unit that remain on hold because Russian officials haven’t agreed.

Grossi reiterated that establishing a protection zone around Zaporizhzhia “remains of paramount importance.”

He said his consultations “are making progress, although not as fast as I would like,” pointing to the need for agreement on technical and other parameters of the zone.

“I hope to be able to do it in a matter of days if possible,” he said. “I will keep on pushing.”

Zaporizhzhia has seen repeated shelling, which Ukraine and Russia accuse each other of carrying out.