UNITED NATIONS, Oct 25 (AFP) - Russia on Tuesday (Oct 25) took its case to the UN Security Council that Ukraine is preparing to use a "dirty bomb" on its own territory, an assertion dismissed by Western and Ukrainian officials as misinformation and a pretext for intensifying the war.

Moscow sent a letter detailing the allegations to the United Nations on Monday, and Russia raised the issue at a closed meeting with the Security Council.

"We're quite satisfied because we raised the awareness," Russia's Deputy UN Ambassador Dmitry Polyanskiy told reporters. "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."

He said the evidence was in intelligence information that had been shared with Western counterparts with the "necessary level of clearance".

Kremlin spokesman Dmitry Peskov on Tuesday repeated Russia's allegations and said the West was foolish to dismiss them.

They follow hints from Moscow that it might be forced to use a tactical nuclear weapon against Ukraine, whose president, Volodymyr Zelenskyy, said the dirty bomb allegation showed Moscow was planning such an attack and seeking to blame Kyiv.

With Ukrainian forces advancing into Russian-occupied Kherson province, threatening a major defeat for Moscow, Russian officials phoned their Western counterparts on Sunday and Monday to air their suspicions.

Russia accused the Kyiv government of ordering two organisations to create a dirty bomb, an explosive device laced with radioactive material, without giving any evidence.

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.

"Russia would be making an incredibly serious mistake for it (to) use a tactical nuclear weapon," US President Joe Biden said on Tuesday. "I'm not guaranteeing you that it's a false flag operation yet, we don't know. But it would be a serious mistake."