Russia will consider the potential use of a so-called “dirty bomb” by Ukraine as an act of nuclear terrorism, Moscow’s ambassador to the United Nations, Vassily Nebenzia, wrote in a letter to UN Secretary-General Antonio Guterres.

The Russian Defense Ministry warned on Monday that Kiev's forces could be planning to detonate a “dirty bomb” in a bid to intimidate the local population, trigger a refugee exodus to the European Union and portray Moscow as “a nuclear terrorist.” The chief of Russia's General Staff, Valery Gerasimov, further relayed these concerns to his US and British counterparts in a series of rare phone conversations, following similar calls by Russian Defense Minister Sergey Shoigu over the weekend.

“We will regard the use of the dirty bomb by the Kiev regime as an act of nuclear terrorism,” Ambassador Nebenzia wrote in a letter to the UN chief and the Security Council members, as reported by Reuters and RIA Novosti on Monday evening.

Reiterating concerns voiced by Russia’s defense officials, Nebenzia has urged Guterres “to do everything in his power to prevent this heinous crime from being carried out,” while calling for a Security Council meeting on the matter. A diplomatic source told RIA Novosti that such a meeting could take place as early as Tuesday.

Moscow has been warning Western nations about Kiev’s alleged plans to use a “dirty bomb” to frame Russia for quite some time, but the West has remained dismissive of such warnings. In a joint statement on Sunday, the foreign ministers of France, the United Kingdom and the United States said they “reject Russia’s transparently false allegations that Ukraine is preparing to use a dirty bomb on its own territory.” NATO Secretary-General Jens Stoltenberg in the meantime dismissed the allegations as “absurd.”

Kiev itself also denied Moscow’s allegations, with Ukrainian President Vladimir Zelensky claiming that “if Russia calls and says that Ukraine is allegedly preparing something, it means one thing: Russia has already prepared all this.”

Kremlin spokesman Dmitry Peskov said that the West’s disbelief “does not mean that the threat of the use of such a ‘dirty bomb’ ceases to exist.”

So-called “dirty bombs” use conventional explosives along with radioactive material. While such weapons could not rival a nuclear warhead in terms of destructive power, they are capable of dispersing a radiation cloud with a radius of several kilometers.

On Sunday, RIA Novosti cited “sources in different countries including Ukraine” as saying that Kiev was in the “final stages” of preparing to detonate “a dirty bomb” on its own territory, in order to accuse Moscow of using weapons of mass destruction in a ploy to “launch a powerful anti-Russia campaign.”