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.

White House national security spokesman John Kirby said Friday that the top national security advisers for the United States, Japan and South Korea met and “expressed grave concern” about North Korea’s troop deployments for potential use with Russia on the battlefield against Ukraine.

Kirby said that the national security advisers from the three countries “call on Russia and the DPRK to cease these actions that only serve to expand the security implications of Russia’s brutal and illegal war beyond Europe and into the Indo-Pacific.”

“It is possible that there are now more than 3,000 troops from North Korea that have been dispatched to Russia for outfitting and for training,” Kirby said on a call with reporters.

Kirby said the U.S. government did not have firm intelligence assessments on where the troops were going “but we believe it is certainly possible” and “perhaps even likely” that some of the North Korean troops would be deployed to the Kursk region of Russia, where Ukraine has held some territory since capturing it in August. But he cautioned that he did not know in what capacity and to what purpose the North Korean troops would be deployed.

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 deployedto 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.