Russia and Ukraine have exchanged allegations about the use of a "dirty bomb" in the conflict between the two countries.

Moscow on Tuesday took its case to the United Nations (UN) Security Council saying that Kyiv is preparing to use a "dirty bomb", while Ukraine's nuclear energy operator Energoatom said on the same day that Russian forces were performing secret work at Zaporizhzhia Nuclear Power Plant - the largest nuclear power plant in Europe.

Moscow on Monday sent a letter detailing the allegations to the UN and raised the issue at a closed Security Council meeting on Tuesday.

France, Britain and the U.S. said the allegations against Ukraine were "transparently false" and Washington warned Russia there would be "severe consequences" for any nuclear use.

"We're quite satisfied because we raised the awareness," Russia's Deputy UN Ambassador Dmitry Polyanskiy told reporters after the council. "I don't mind people saying that Russia is crying wolf if this doesn't happen because this is a terrible, terrible disaster that threatens potentially the whole of the Earth."

Polyanskiy said the evidence was intelligence that had been shared "in our telephone conversation with counterparts who have the necessary level of clearance."

Technically known as radiological dispersion devices, dirty bombs are relatively primitive, imprecise weapons. Dirty bombs don't have the devastating destruction of a nuclear explosion but could expose broad areas to radioactive contamination.

Ukraine also dismissed Russia's claim as an attempt to distract attention from the alleged Russian plans to detonate a dirty bomb at Zaporizhzhia Nuclear Power Plant. 

Energoatom, the Ukrainian state enterprise that operates the country's four nuclear power plants, said Russian forces have carried out secret construction work over the last week at the occupied Zaporizhzhia Nuclear Power Plant in Ukraine.

Russian officers controlling the area won't give access to Ukrainian staff running the plant or monitors from the International Atomic Energy Agency (IAEA) -UN's atomic energy watchdog - that would allow them to see what the Russians are doing, Energoatom said on Tuesday in a statement.

Energoatom called on the IAEA to assess what was going on.

IAEA Director General Rafael Mariano said on Monday that the UN nuclear watchdog was preparing to send inspectors to two unidentified Ukrainian sites at Kyiv's request, both already subject to its inspections.

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

As the Russia-Ukraine conflict entered its ninth month, Russian forces are digging in for the "heaviest of battles" in the strategic southern region of Kherson, Reuters reported, citing a senior Ukrainian official.

Russian forces in the region have been driven back in recent weeks and risk being trapped against the west bank of the Dnipro river, where the provincial capital of Kherson was swiftly captured by Russian forces after the Russia-Ukraine conflict began on February 24. 

In the Mykolaiv region north and west of Kherson city, artillery duels raged throughout Tuesday, according to a post from the frontline on Rybar, a pro-Russian channel on Telegram.

In Ishchenka district north of Kherson, Ukrainian forces tried to consolidate their positions, but were forced back to earlier lines, the post said. It said the Ukrainian military was preparing for an advance along the entire length of the frontline.

(With input from agencies)