The UN nuclear watchdog said it was preparing to send inspectors to two unidentified Ukrainian sites at Kyiv's request.

Russia sent a letter on its assertion about Kyiv to the United Nations late on Monday, and diplomats said Moscow planned to raise the issue with the Security Council at a closed meeting the following day.

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"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 told UN Secretary-General Antonio Guterres and the Security Council in the letter, seen by the media.

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.

The foreign ministers of France, Britain and the United States rejected Moscow's allegations as "transparently false" and reiterated their support for Ukraine.

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

Later, the United States issued a warning to Russia.

"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."

State Department spokesman Ned Price.

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 it was preparing to send inspectors to two unidentified Ukrainian sites at Kyiv's request, in an apparent reaction to the Russian talk of a "dirty bomb". 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.

US officials 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 spokesman John Kirby told reporters.

Ukrainian President Volodymyr Zelensky said Russia's accusation was a sign it was planning such an attack and would blame Ukraine.

President Volodymyr Zelensky listens during a meeting during a working trip to the Zaporizhzhia region. AFP

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

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

German President Frank-Walter Steinmeier arrived in Ukraine on Tuesday on his first visit since Russia invaded on Feb. 24 and would meet Zelensky, German broadcaster ntv reported.

Reuters