UNITED NATIONS, Oct 24 (AFP) - Russia intends to raise at the United Nations Security Council on Tuesday its accusation that Ukraine is planning a "dirty bomb" attack and has urged U.N. chief Antonio Guterres to do all he can to "prevent this heinous crime from happening."

With Ukrainian forces advancing into the strategic Russian-occupied province of Kherson, top Russian officials phoned Western counterparts on Sunday and Monday to tell them Moscow suspected Ukraine of planning to use a so-called "dirty bomb" laced with nuclear material.

Ukrainian President Volodymyr Zelenskiy has responded by accusing Russia of planning such an attack itself to blame on Ukraine. Western countries also rejected Russia's allegation as a pretext for intensifying the eight-month-long war, which began in February when Russian forces invaded neighboring Ukraine.

"We will regard the use of the "dirty bomb" by the Kiev regime as an act of nuclear terrorism," Russia's U.N. Ambassador Vassily Nebenzia wrote in a letter - seen by Reuters - to Guterres and the Security Council on Monday.

"We urge the Western countries to exert their influence on the regime in Kiev to abandon its dangerous plans threatening international peace and security," he wrote. "We call on the Secretary-General of the United Nations to do everything in his power to prevent this heinous crime from happening."

Russia has told council counterparts it will bring up the issue during a closed-door meeting on Tuesday, diplomats said.

Britain's mission to the United Nations posted on Twitter late on Monday: "Reminder: Ukraine has no nuclear weapons."

Earlier on Monday, U.N. spokesman Stephane Dujarric said: "All parties should avoid any actions that could lead to miscalculation and escalation of what's already a devastating conflict."

The U.N. Security Council has met dozens of times on Ukraine but is unable to take any meaningful action because Russia is a veto-power - along with the United States, China, Britain and France.

Russia has been diplomatically isolated at the world body. Earlier this month three-quarters of the 193-member General Assembly - 143 countries - voted to condemn Russia's "attempted illegal annexation" of four partially occupied regions in Ukraine and called on all countries not to recognize the move.