Ukraine and its Western allies on Monday claimed that Russia was plotting to use the threat of a "dirty bomb" as a pretext for escalation in Ukraine.

Moscow, in the meanwhile, evacuated civilians from the southern city of Kherson in anticipation of a major battle as Ukrainian forces advanced into the Russian-occupied province.

Russia's Defense Minister Sergei Shoigu phoned Western counterparts earlier Sunday to tell them Moscow suspected Kyiv of planning to use a so-called dirty bomb, one laced with nuclear material.

Later Monday, Moscow claimed that Ukraine had nearly completed developing the bomb. "According to the information we have, two organizations in Ukraine have specific instructions to create a so-called 'dirty bomb.' This work is in its final stage," Lt. Gen. Igor Kirillov said in a statement.

He said Kyiv was planning to accuse Russia of "using weapons of mass destruction in Ukraine and thus launch a powerful anti-Russian campaign in the world."

However, the foreign ministers of France, Britain and the U.S. in a joint statement had earlier said they all rejected the allegations and reaffirmed their support for Ukraine against Russia.

"Our countries made clear that we all reject Russia’s transparently false allegations that Ukraine is preparing to use a dirty bomb on its own territory," they said. "The world would see through any attempt to use this allegation as a pretext for escalation."

In an overnight address, Ukrainian President Volodymyr Zelenskyy said the Russian accusation was a sign Moscow was planning such an attack itself and would blame Ukraine.

"If Russia calls and says that Ukraine is allegedly preparing something, it means one thing: Russia has already prepared all this," Zelenskyy said.

"So when today the Russian Minister of Defense organizes a phone carousel and calls foreign ministers with stories about the so-called 'dirty' nuclear bomb, everyone understands everything well. Understands who is the source of everything dirty that can be imagined in this war."

Evacuation

Russia, in the meanwhile, has ordered civilians to evacuate territory it controls on the western bank of the Dnipro River, where Ukrainian forces have been advancing since the start of this month shortly after Moscow claimed to have annexed the area.

A Russian defeat there would be one of Moscow's biggest setbacks of the war. Kherson's regional capital is the only big city Russia has captured intact since the invasion in February, and its only foothold is on the west bank of the Dnipro, which bisects Ukraine. The province controls the gateway to Crimea, the peninsula Russia seized and claimed to annex in 2014.

The Russian-installed authorities in Kherson announced on Monday that men who stay behind would have the option of joining a military self-defense unit. Kyiv accuses Russia of press-ganging men in occupied areas into military formations, a war crime under the Geneva Conventions.

Since his forces suffered major battlefield defeats in September, President Vladimir Putin has escalated the war, calling up hundreds of thousands of reservists, announcing the annexation of occupied territory and repeatedly threatening to use nuclear weapons to defend Russian land.

This month, Russia has started a new campaign using long-range cruise missiles and Iranian-made drones to attack Ukraine's electric power and heat ahead of winter.

Russian state television is filled with talk shows featuring pundits who openly cheer attacks on Ukrainian civil infrastructure and call for ever-tougher measures to eliminate what they describe as an illegitimate Ukrainian state.

On Monday, Russian state TV presenter Anton Krasovsky apologized for remarks in which he called for Ukrainian children to be drowned in rivers and burned alive in huts with the doors nailed shut. He also joked that Ukrainian grandmothers were saving their funeral funds to pay Russian soldiers to rape them.

"Well, it happens: you're on air, you get carried away. And you can't stop," Krasovsky said, adding he was "really embarrassed".

Krasovsky was suspended from Russia's state-funded international channel RT, and Russia's Investigative Committee said it had ordered a report into his "sharp comments".

Ukraine's Foreign Minister Dmytro Kuleba said Krasovsky would be prosecuted someday for promoting genocide, and called on countries to ban RT.