North Korean troops deployed to Russia have already been spotted on the battlefields of the Ukraine-Russia war, the Ukrainian defense intelligence service said Thursday.

U.S. and Ukrainian officials have watched with alarm as North Korean forces have been seen training at military bases in the Far East. The Biden administration says at least 3,000 North Korean troops have been dispatched to aid in Russia’s invasion of its smaller neighbor, but Ukrainian government officials have said the contingent may be four times that number.

Ukraine’s intelligence service said in a social media posting Thursday that at least some North Korean forces had already been spotted in the combat zone near the front lines. The troops were said to be deployed in Kursk, the Russian border region that was the target of a surprise invasion last month by Ukrainian troops.

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Russian and Ukrainian forces are battling along a more than 600-mile front in eastern and southern Ukraine, with Moscow’s forces making some territorial gains in recent months.

Ukrainian President Volodymyr Zelenskyy said this week that the regime of North Korean leader Kim Jong-un had already sent tactical military specialists and officers into Russian-occupied areas of his country, which the U.S. has yet to confirm.

The U.S. and South Korea fear that the North Korean contingent will provide fresh fighting forces for the strapped Russian army and that Mr. Kim may receive economic aid and sophisticated military technology from the Kremlin as payment for agreeing to provide troops.

Russian President Vladimir Putin, hosting the multinational BRICS summit in Kazan this week, was coy when asked by reporters Thursday about satellite images apparently showing North Korean troops on the move inside Russia.

“Images are a serious thing,” Mr. Putin said. “If there are images, then they reflect something.”

North Korea has denied it has sent any troops to Russia or that there are plans for North Korean troops to take part in the Ukraine war. Mr. Putin says it is the U.S. and its Western allies who are responsible for escalating the conflict and has contended that Western troops are already fighting alongside Ukrainian forces in the war.