Russia doubled down on a warning that Ukraine is preparing to use a "dirty bomb" on its own territory, an assertion dismissed by the West and Kyiv as false, and was expected to bring the issue to the United Nations Security Council later on Tuesday.

Moscow sent a letter detailing its allegations about Kyiv to the U.N. late on Monday, and diplomats said Russia planned to raise the issue at a closed meeting with the Security Council on Tuesday.

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

Ukrainian President Volodymyr Zelenskyy has said Russia's accusation was a sign that Moscow – which has threatened to use a tactical nuclear weapon in Ukraine – was planning such an attack and was preparing to shift the blame to Ukraine.

With Ukrainian forces advancing into Russian-occupied Kherson province, top Russian officials had phoned their Western counterparts on Sunday and Monday to air their suspicions.

France, Britain and the United States said the allegations were "transparently false" and Washington warned Russia there would be "severe consequences" for any nuclear use, while saying there were no signs of that yet.

"There would be consequences for Russia whether it uses a dirty bomb or a nuclear bomb," U.S. State Department spokesperson Ned Price said. The White House said there was no indication Russia had decided 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 spokesperson John Kirby told reporters.

Russia's Defense Ministry said the aim of a "dirty bomb" attack by Ukraine would be to blame Moscow for the resulting radioactive contamination, which Russia had begun preparing for.

The U.N. nuclear watchdog said it was preparing to send inspectors to two unidentified Ukrainian sites at Kyiv's request, both already subject to its inspections, in an apparent response to Russia's "dirty bomb" claim.

Russia's state news agency RIA has identified what it said were the two sites involved in the operation – the Eastern Mineral Enrichment Plant in the central Dnipropetrovsk region and the Institute for Nuclear Research in Kyiv.

More help for Ukraine?

German President Frank-Walter Steinmeier arrived in Ukraine on Tuesday on his first visit since Russia invaded on Feb. 24 and would meet Zelenskyy, German broadcaster NTV reported, as Berlin hosted what it said was a conference on a "Marshall Plan" to rebuild Ukraine.

Zelenskyy told the conference via video link that Russian rockets and Iranian-made drones had destroyed more than a third of his country's energy sector, but that Kyiv had yet to receive "a single cent" towards a fast recovery plan worth $17 billion.

European Commission chief Ursula von der Leyen cited the World Bank as putting the cost of rebuilding at 350 billion euros ($345 billion).

In southern Ukraine, 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 after Russia claimed to have annexed the area.

A defeat for Russia there would be one of its biggest setbacks since its invasion.

Ukraine's military said Russian-installed authorities in Kherson were evacuating banks, administrative facilities, and emergency service and medical personnel, while funding for schools and school meals had been stopped. Equipment used by internet service providers had been stolen and robberies of residents and looting had increased, it said.

Reports could not be verified.

Kherson's regional capital is the only big city Russia has captured intact since the start of the 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.

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 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.