A "dirty bomb" is a conventional bomb laced with radioactive, biological or chemical materials which get disseminated in an explosion.

The term is often used interchangeably with radiological dispersal device (RDD), a bomb where radioactive materials are used.

In a joint statement, the US, France and Britain -- three of the other nuclear powers on the United Nations Security Council -- said Sunday that Russia's claims were "transparently false".

They and Kyiv suspect that Russia might itself use a dirty bomb in a "false flag" attack, possibly to justify use of conventional nuclear weapons by Moscow as it finds itself on the back foot in eastern and southern Ukraine.

The Russian allegation has nevertheless prompted rare communication with the West, with Moscow's chief of staff Valery Gerasimov speaking on Monday to his US counterpart Mark Milley for the first time since May.

Early Tuesday, Ukraine's army chief Valeriy Zaluzhnyi said he too had spoken to Milley.

"I have assured that Russian accusations (about) plans of Ukraine to use a 'dirty bomb' are disinformation'," a post on his Facebook page read.

"The Armed Forces of Ukraine have nothing to do with weapons of mass destruction of any kind."

Easier to make and less destructive than a nuclear bomb, a dirty bomb's effect would contaminate a specific area, and the people there, either by direct radiation or through the inhalation or ingestion of contaminated substances.

"A dirty bomb is not a weapon of mass destruction but a 'weapon of mass disruption', where contamination and anxiety are the major objectives," said the United States Nuclear Regulatory Commission, an independent agency.

- 'Escalate to avoid defeat' -

"The Russians are turning to this logic of escalation because of the very real difficulties on the front line" battered by Ukraine's counter-offensives eight months after their invasion began, one Western military source told AFP on condition of anonymity.

In September, Moscow's forces lost thousands of square kilometres of territory in Ukraine's northeast, while they are now retreating in the southern Kherson region.

For the Russians, "conventional and nuclear weapons back each other up," the military source said.

"If they're out of balance, you find yourself having to escalate to avoid a tactical defeat on the battlefield that could become a strategic defeat," they added.

Some commentators have pointed out a similar grab at justifications for a nuclear strike in March.

Early on in its assault on Ukraine, after a lightning offensive to take Kyiv had failed, Moscow accused its neighbour of operating laboratories developing chemical and biological weapons.

- 'Nobody would blame Ukraine' -

Most analysts believe that there is little chance a false flag dirty bomb attack could really be used as a pretext for a nuclear detonation.

Western intelligence officials have not observed any change in the posture of Russia's nuclear forces.

What's more, "if this was a false flag event, we would know it instantly, nobody would blame Ukraine," said William Alberque, an arms control expert at the London-based International Institute for Strategic Studies.

"Radiological weapons are so identifiable, so scrutinised. You can make a chemical weapon from scratch," but "nuclear material has a fingerprint" based on the facilities used to create it, he added.

Russia's warnings about a dirty bomb detonation are "just noise" for now, Alberque said.

But in future, "they'll say Ukraine was deterred from doing it and they'll take credit for it," he suggested.

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