The Russian military has assembled a force of 50,000 soldiers, including North Korean troops, as it prepares to begin an assault aimed at reclaiming territory seized by Ukraine in the Kursk region of Russia, according to U.S. and Ukrainian officials.

       A new U.S. assessment concludes that Russia has massed the force without having to pull soldiers out of Ukraine’s east — its main battlefield priority — allowing Moscow to press on multiple fronts simultaneously.

       Russian troops have been clawing back some of the territory that Ukraine captured in Kursk this year. They have been attacking Ukrainian positions with missile strikes and artillery fire, but they have not yet begun a major assault there, U.S. officials said.

       Ukrainian officials say they expect such an attack involving the North Korean troops in the coming days.

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       For now, the North Koreans are training with Russian forces in the far western part of Kursk.

       The Russian-North Korean offensive looms as President-elect Donald J. Trump prepares to re-enter office with a stated goal of ending the war quickly. Mr. Trump has said little about how he would settle the conflict, but Vice President-elect JD Vance has outlined a plan that would allow Russia to keep the territory it has seized in Ukraine.

       Some U.S. military and intelligence officials have grown more pessimistic about Ukraine’s overall prospects, noting that Russia has steadily gained ground, both in Kursk and in eastern Ukraine. Officials say the setbacks are partly a result of Ukraine’s failure to solve critical shortfalls in troop strength.

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