The U.N.’s nuclear watchdog said Thursday that it had inspected three Ukrainian facilities and found no evidence of illegal nuclear activity, debunking claims Russian officials made last week that Ukraine was using the sites to prepare a “dirty bomb.”

Dirty bombs are improvised bombs that use conventional high explosives to spread radioactive material into the surrounding area. President Vladimir Putin of Russia joined his senior military leaders in making the assertion that Ukraine was planning to use one.

But the top diplomats of the United States, Britain and France have firmly rejected the Russian claims, which were unaccompanied by any evidence. Last week, the diplomats issued a rare joint statement saying that the Kremlin could be using the false claim as a pretext to escalate its war on Ukraine.

Ukraine also rejected Russia’s claim and, in a bid to show that it was not producing such a weapon, invited the U.N. watchdog, the International Atomic Energy Agency, to inspect the three facilities featured in Russia’s accusation. The sites are a mine in the center of the country, a machine building plant in Dnipro province and a nuclear research institute in the capital, Kyiv.

“Our technical and scientific evaluation of the results we have so far did not show any sign of undeclared nuclear activities and materials at these three locations,” the agency’s director general, Rafael Mariano Grossi, said in a statement Thursday, following the agency’s inspection.

Fears of dirty bombs have emerged from time to time in the nuclear age, stirring public fears. In the wake of the Sept. 11, 2001, attacks, government officials occasionally warned that terrorists could build a dirty bomb with radioactive materials used in many commercial industries.

Ukraine has been keen to show that it is cooperating with international authorities like the IAEA, which has warned about the perils of dirty bombs. Such bombs, although they lack the explosive power of a nuclear weapon, can contaminate localized areas — a few city blocks or a village — and make them uninhabitable.

Russia’s claim came amid heightened fears in the West that Moscow could be seeking a pretext to escalate the war following a recent series of battlefield losses in Ukraine’s northeast and a Ukrainian counteroffensive in the southern, strategically important Kherson region.

A day before Putin made his unsubstantiated accusations, President Joe Biden warned him that it would be “an incredibly serious mistake” to use a tactical nuclear weapon in the conflict.

This article originally appeared in The New York Times.