NATO Allies Warn Russia Against Dirty Bomb Plot

Mr. Shoigu told them that the war in Ukraine was moving toward a more dangerous phase and that Kyiv might soon deploy a dirty bomb, which combines conventional explosives with radioactive materials such as cesium or cobalt and would contaminate territory without immediately killing a large number of people.

Western officials sought Monday to decipher Russia’s motives in making the allegation, and dismissed it as absurd and false. Ukraine denied the allegation, and officials in the U.S. and Ukraine warned that Russia could be signaling that it is preparing to use such weapons itself in a false-flag operation and then use it as an excuse for deploying a nuclear weapon.

“We have seen in the past that the Russians have, on occasion, blamed others for things that they were planning to do,” National Security Council Coordinator for Strategic Communication John Kirby told reporters.

Gen. Mark Milley, the Chairman of the Joint Chiefs of Staff, discussed the claim with his Russian counterpart on Monday, U.S. defense officials said.

Western experts said using a dirty bomb would make little sense for Ukraine. It would cause a relatively low level of destruction that wouldn’t alter the shape of the conflict and would risk a far more devastating Russian nuclear response.

“A dirty bomb would be an ineffective battlefield weapon. Generally speaking, a dirty bomb is a crude device that seeks to spread radioactive contamination over a relatively small area of land—several small blocks,” said Scott Roecker, vice president for nuclear materials security at the Nuclear Threat Initiative, a nonprofit global-security think tank. “A dirty bomb is more a psychological weapon than a weapon that would cause mass destruction. It wouldn’t change the direction of the war,” he said.

Lacking an easy off-ramp from the war, Russian President Vladimir Putin has lately sought to buy time and undermine Western support for Ukraine by threatening apocalyptic escalation and mobilizing 300,000 Russian reservists, whom Moscow is now funneling to the front. A Ukrainian counteroffensive is threatening the Russian-held provincial capital of Kherson, which if taken would be Moscow’s most serious territorial setback of the war.

Russia has since the beginning of its invasion made accusations against Kyiv that the U.S. and its allies have dismissed as Moscow’s attempt to deflect attention from its own brutality.

Those include allegations that Ukraine is trying to acquire nuclear warheads, that it is preparing chemical weapons and that the U.S. and Kyiv have been operating a network of bioweapon laboratories in Ukraine, with the personal assistance of President Biden and investments from his son Hunter Biden and investor George Soros.

Western officials say that the Russian signaling Sunday was ominous because it was phoned in by Russia’s defense minister, who has seldom engaged with the U.S. or other allies. Before Sunday, Mr. Shoigu had only spoken twice to his U.S. counterpart, Defense Secretary Lloyd Austin, since the beginning of the war. The call between Mr. Austin and his defense counterpart was arranged Saturday, a U.S. official said.

In their response to Mr. Shoigu, Western defense ministers appeared to take seriously the threat of an escalation in Ukraine, including use of nuclear materials, said Andrew Weiss, vice president at the Carnegie Endowment for International Peace in Washington. “Policy makers also have an uncomfortable question,” he said. “Is it simply another Russian ruse or is Russia foreshadowing something that previously seemed unthinkable?”

Mr. Shoigu’s calls sparked a flurry of trans-Atlantic consultations. After speaking with Mr. Shoigu on Sunday, Mr. Austin called Ben Wallace, the U.K. secretary of state for defense. U.S. Secretary of State Antony Blinken spoke to Ukraine’s foreign minister and told him that the U.S. rejected Mr. Shoigu’s “transparently false” allegation that Ukraine was preparing a dirty bomb, according to a U.S. statement.

Ukrainian Foreign Minister Dmytro Kuleba said Monday that he had spoken with the head of the United Nations’ International Atomic Energy Agency, Rafael Grossi, and the agency had agreed to send experts to sites in Ukraine which Russia “deceitfully claims to be developing a dirty bomb.”

“We have nothing to hide,” Mr. Kuleba said on Twitter.

An IAEA spokesman said Mr. Kuleba had invited inspectors to Ukraine to examine the Russian claims and that the agency would announce its plans soon. As part of its core global-safeguards work, the agency already reviews the risk of nuclear material being diverted for military purposes in Ukraine. Mr. Grossi was scheduled to meet Mr. Blinken in Washington on Monday in Washington.

Mr. Shoigu’s allegations aren’t the first time Russia has claimed Ukraine is working on nuclear material to be potentially used in the war. In the first days of the conflict, Russian Foreign Minister Sergei Lavrov alleged that Kyiv had embarked on plans to acquire its own nuclear weapons and claimed that Ukraine still had old Soviet technologies and the means to deliver the weapons.

Mr. Lavrov’s claims were dismissed days later by Mr. Grossi of the IAEA, who said there was no evidence that would raise questions about Ukraine’s “nonproliferation credentials.”

—Nancy A. Youssef, William Mauldin and Vivian Salama contributed to this article.

Write to Alan Cullison at alan.cullison@wsj.com and Laurence Norman at laurence.norman@wsj.com

Corrections & Amplifications
Antony Blinken is the U.S. secretary of state. An earlier version of this article incorrectly spelled his first name as Anthony. (Corrected on Oct. 24)