Ukraine is bracing for assaults involving North Korean soldiers who arrived last week in Russia’s western Kursk region, where they are expected to support Moscow’s efforts to dislodge Ukrainian forces who invaded in August.

The NATO secretary general, Mark Rutte, on Monday confirmed that North Korean troops had been deployed in the Kursk region, saying it represented “a dangerous expansion” of the war. Ukrainian and American officials said last week that several thousand North Korean troops had arrived in the area.

On Friday, President Volodymyr Zelensky of Ukraine said the North Korean troops were expected to enter combat operations early this week.

A Ukrainian soldier fighting in the Kursk area said he had been warned by his commanders that an assault could be imminent. “They have warned us about an attack in the near future,” Lt. Col. Artem Kholodkevych, the deputy commander of Ukraine’s 61st Mechanized Brigade said on Saturday by text message. “Probably in the next few days.”

The Ukrainian Army has also issued a Ukrainian-Korean phrase book for its troops to address North Korean soldiers and urge them to surrender, according to a Ukrainian officer who spoke on condition of anonymity to discuss sensitive matters.

Russian forces have been battling to reclaim hundreds of square miles of land seized by Ukrainian troops in the Kursk region during a surprise cross-border offensive in August. In recent weeks, Russia has regained several villages, and military experts say the deployment of North Korean soldiers could bolster Russian counterattacks.