U.S. President Joe Biden warned Russia on October 25 that the use of a tactical nuclear weapon would be an "incredibly serious mistake" after Moscow claimed Ukraine is preparing to use a so-called dirty bomb on its own territory.
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Biden said he has spent "a lot of time" dealing with Russia's claim, which Moscow raised over the weekend, drawing immediate dismissal from the United States and other countries that have backed Ukraine.
"Let me just say Russia would be making an incredibly serious mistake were it to use a tactical nuclear weapon." Biden told reporters. "I cannot guarantee you that it is a false flag operation yet. We don’t know. But it would be a serious mistake."
Ukraine and its allies suspect Russia might have made the claim to set up a "false flag" attack in which it would use a so-called dirty bomb but would blame the attack on Ukraine and use it to justify the use of conventional nuclear weapons by Moscow.
Russian Defense Minister Sergei Shoigu presented no evidence for the claim when he spoke on October 23 with his counterparts from several NATO countries, including Britain, France, and the United States, who dismissed the claim after the series of calls.
Kremlin spokesman Dmitry Peskov told reporters on October 25 that Washington's refusal to take note of Russia's warning was "inappropriate given the seriousness of the threat we are talking about."
A dirty bomb would use a conventional warhead to create an explosion that would spread radioactive, biological, or chemical materials over an area.
Moscow took its accusations against Ukraine to the UN Security Council on October 25. The country's UN ambassador, Dmitry Polyanskiy, said afterward that Russia was "satisfied because we raised the awareness."
Speaking to reporters, he added: "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."
British Ambassador to the UN James Kariuki called Russia's accusation "transparently false."
No new evidence was presented to the Security Council during its private meeting, Kariuki said.
"Ukraine has been clear: It's got nothing to hide," he added.
WATCH: Speaking to Current Time in Riga on October 22, Latvian Defense Minister Artis Pabriks said Russian President Vladimir Putin cannot change the course of war in Ukraine by dropping nuclear bombs.
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The International Atomic Energy Agency (IAEA) said earlier on October 25 that it is preparing to send inspectors to two Ukrainian sites in the coming days in reaction to Ukraine's request for an inspection following Russia's claims.
Ukrainian Foreign Minister Dmytro Kuleba on October 24 invited the IAEA to "urgently send experts to peaceful facilities in Ukraine which Russia deceitfully claims to be developing a dirty bomb."
Kuleba said Ukraine has always been transparent and has "nothing to hide."
Enerhoatom, Ukraine’s nuclear energy operator, issued a statement on October 24 voicing its concern that Russia’s statements “may indicate that Russia is preparing an act of nuclear terrorism.”
Russian troops have occupied Ukraine’s Zaporizhzhya nuclear power plant, Europe's largest, since March. It is still run by Ukrainian engineers, though Russia claimed after its illegal annexation of the Zaporizhzhya region that it is on Russian territory.
Enerhoatom said that Russian forces have carried out unauthorized, secret construction work over the last week at the plant in the area of the spent nuclear fuel storage facility.
Russian officers controlling the area won’t give access to Ukrainian staff or monitors from the IAEA that would allow them to see what they are doing, the operator said.
Enerhoatom added that it “assumes” the Russians “are preparing a terrorist act using nuclear materials and radioactive waste stored” at the plant.