North Korean troops are poised to be deployed by Russia on the battlefield in Ukraine as early as this weekend, Ukrainian President Volodymyr Zelenskyy claimed Friday.

Western officials have warned that North Korean units joining the fight would stoke the almost three-year war and bring geopolitical consequences as far away as the Indo-Pacific region.

The possibility has alarmed leaders and deepened diplomatic tensions around the war. A senior official in the Ukrainian presidential office told The Associated Press on Friday that Zelenskyy had canceled a planned visit to Kyiv by U.N. Secretary-General Antonio Guterres.

The official, who spoke on condition of anonymity because he was not authorized to speak publicly about the matter, said the visit was supposed to come after this week’s summit in the Russian city of Kazan of the BRICS bloc of developing economies, which Guterres attended.

A photograph of Guterres shaking hands with Russian President Vladimir Putin at the summit triggered an outcry in Ukraine.

Zelenskyy, in a post on Telegram, said Ukrainian intelligence had determined that “the first North Korean military will be used by Russia in combat zones” between Sunday and Monday.

He said on Telegram that the deployment was “an obvious escalating move by Russia.” He didn’t provide any further details, including where the North Korean soldiers may be sent.

Russia has been conducting a ferocious summer campaign along the eastern front in Ukraine, gradually compelling Kyiv to surrender ground. But Russia has struggled to push Ukrainian forces out of its Kursk border region following an incursion almost three months ago.

North Korean units were detected on Wednesday in Kursk, according to Ukraine’s Main Intelligence Directorate, known by its acronym GUR.

The soldiers had undergone several weeks of training at bases in eastern Russia and had been equipped with clothes for the upcoming winter, GUR said in a statement late Thursday.

It estimated the number of North Korean soldiers sent by Pyongyang to Russia at around 12,000, including some 500 officers and three generals.

GUR provided no evidence for its claims.

Dutch Defense Minister Ruben Brekelmans said Friday on the social platform X that intelligence reports indicated the North Korean soldiers “will probably first be deployed in Kursk.”

The deployment of North Korean forces under a military pact between Moscow and Pyongyang brings a new dimension to the conflict, which is Europe’s biggest war since World War II and has cost tens of thousands of lives on both sides, including many civilians.

The U.S. said Wednesday that 3,000 North Korean troops have been deployed to Russia and are training at several locations, calling the move very serious.

Zelenskyy said a week ago that his government has intelligence information that 10,000 troops from North Korea are being readied to join Russian forces fighting against his country. He said that a third nation wading into the hostilities would turn the conflict into a “world war.”

North Korea had already been supplying ammunition to Russia under a defense pact, but putting boots on the ground could severely complicate a war that has inflamed international politics, with most Western countries supporting Kyiv.

Putin, meanwhile, has looked for support among BRICS countries.

He has neither confirmed nor denied that North Korean troops were in Russia.