The United States warned on Monday there would be severe consequences if Russia used a nuclear weapon in Ukraine, while Russia took its claim that Kyiv was planning to deploy a "dirty bomb" to the United Nations.

With Ukrainian forces advancing into Russian-occupied Kherson province, top Russian officials phoned Western counterparts on Sunday and Monday to tell them of Moscow's suspicions.

The foreign ministers of France, Britain and the United States rejected the allegations as "transparently false" and reaffirmed their support for Ukraine in a joint statement.

"The world would see through any attempt to use this allegation as a pretext for escalation," they said.

Later, the United States issued a warning to Moscow.

"We've been very clear with the Russians ... about the severe consequences that would result from nuclear use," State Department spokesman Ned Price said. "There would be consequences for Russia whether it uses a dirty bomb or a nuclear bomb."

Russia said it had no intention of using a nuclear weapon itself. It sent a letter on its claims about Kyiv to the UN on Monday and will raise the issue with the Security Council at a meeting on Tuesday.

"We will regard the use of the dirty bomb by the Kyiv regime as an act of nuclear terrorism," Russia's UN Ambassador Vassily Nebenzia wrote in the letter to UN Secretary-General Antonio Guterres and the Security Council.

Russian military Chief of General Staff Valery Gerasimov spoke to the chairman of the US Joint Chiefs of Staff, Mark Milley, on Monday, Russia's RIA news agency reported.

US officials said there was no indication Moscow had made the decision to use a dirty bomb or any nuclear weapon.

"We continue to see nothing in the way of preparations by the Russian side for the use of nuclear weapons," White House national security spokesman John Kirby told reporters.

Russia's defence ministry said the aim of a "dirty bomb" attack by Ukraine would be to blame Russia for the resulting radioactive contamination. The ministry has begun preparing for such a scenario, it said, readying forces and resources "to perform tasks in conditions of radioactive contamination."

The UN nuclear watchdog said on Monday it was preparing to send inspectors in the coming days to two Ukrainian sites at Kyiv's request, in an apparent reaction to the Russian "dirty bomb" claims. It said both sites were already subject to its inspections and one was inspected a month ago.

Russia's state news agency RIA had earlier identified what it said were the two sites involved in the operation - the Eastern Mineral Enrichment Plant in central Dnipropetrovsk region and the Institute for Nuclear Research in Kyiv. The IAEA statement did not name the facilities it would inspect.

Ukrainian President Volodymyr Zelenskiy 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," Zelenskiy said in an overnight address.

Ukrainian foreign minister Dmytro Kuleba said late Monday he had a detailed discussion with US Secretary of State Antony Blinken on "ways to end Russia's nuclear blackmail."

Evacuation

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

A Russian defeat there would be one of Moscow's biggest setbacks yet since its invasion eight months ago.

Kherson's regional capital is the only big city Russia has captured intact since its Feb 24 invasion, and its only foothold 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-defence unit. Kyiv accuses Russia of press-ganging men in occupied areas into military formations, a war crime under the Geneva Conventions.

Kyrylo Budanov, Ukraine's military spy chief, said Russian forces were preparing to defend Kherson city, not retreat.

"They are creating the illusion that all is lost. Yet at the same time they are moving new military units in and preparing to defend the streets of Kherson," he told the Ukrainska Pravda online media outlet.

Since Russia's 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 started a new campaign using long-range cruise missiles and Iranian-made drones to attack Ukraine's energy infrastructure before winter sets in.

Russian state television is filled with talk shows featuring pundits who are openly cheering attacks on Ukrainian civil infrastructure.

On Monday, Russian state TV presenter Anton Krasovsky apologised 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.

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".

- Reuters