50,000 Russian and North Korean Troops Mass Ahead of Attack, U.S. Says

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.

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.

President Biden has been deeply supportive of Ukraine, pushing Congress to approve billions of dollars in aid and having the U.S. military and spy agencies provide critical intelligence to help Kyiv fight the war.

One Western official said Ukraine’s surprise incursion into Kursk in August thinned out its forces across the battlefield in eastern Ukraine, leaving them vulnerable to Russian advances. But that official, and U.S. officials, said Ukraine still had a strong defense in Kursk and might be able to hold, at least for a time.

The officials interviewed for this article spoke on the condition of anonymity to discuss sensitive intelligence assessments and offer a candid appraisal of Ukraine’s battlefield prospects.

Western and Ukrainian officials say the arrival of North Korean forces is a major escalation after more than two years of war.

North Korea has sent more than 10,000 troops to fight with Russia in Kursk, American officials say. The troops are wearing Russian uniforms and have been equipped by Moscow, but they will probably fight in their own discrete units, U.S. defense officials said.

Ukrainian officials said Moscow had supplied the North Korean forces with machine guns, sniper rifles, antitank missiles and rocket-propelled grenades.

Russia has been training the North Koreans in artillery fire, basic infantry tactics and, critically, trench clearing, American officials said. That training suggests that at least some of the North Korean forces will be involved in frontal assaults on Ukraine’s dug-in defensive positions.

“We fully expect that D.P.R.K. soldiers could be engaged in combat,” Sabrina Singh, the deputy Pentagon press secretary, said on Thursday, using the initials of North Korea’s formal name, the Democratic People’s Republic of Korea.

U.S. officials are not sure what constraints the government of President Kim Jong-un has put on the use of its forces. However, American officials expect them to be directly involved in the fighting.

A Ukrainian official said the North Korean forces had been divided into two groups, an assault unit and a support unit, which will help provide security inside the territory recaptured from Ukrainian forces.

North Korea has a large army but, unlike Russia, has not been involved in ground combat for decades. The troops North Korea is deploying, however, are considered its best, drawn from the 11th Corps, home to the country’s special operations soldiers.

The Ukrainians captured hundreds of square miles of territory with little opposition, but Russia has slowly chipped away at those gains — reclaiming roughly half the seized territory — and now appears ready to conduct a much larger-scale operation.

American officials believe that the Ukrainian troops will prove difficult to dislodge, and that the Russian and North Korean forces will probably take heavy losses, similar to what Russia has suffered in eastern Ukraine. U.S. and British military analysts put the current number of Russian troop deaths and injuries at an average of more than 1,200 a day.

The North Koreans will be fighting as light infantry, without the benefit of armored vehicles. And current Ukrainian tactics of artillery fire and drone attacks have proved devastating to unprotected Russian troops.

That said, if Russia gains momentum, it may not stop at its border but might try to drive Ukrainian forces back farther. It is not clear if the North Korean government will authorize its forces to conduct sustained operations in Ukraine or if they are intended only for the Kursk counteroffensive, according to U.S. defense officials. Some American officials believe that North Korea could order its troops to stop at the border while the Russian forces press deeper into Ukraine.

U.S. officials said they did not know how effective the North Koreans would be, considering their lack of ground combat experience.

George Barros, an analyst at the Institute for the Study of War, said that in spite of that inexperience, the North Korean forces are well organized. “The one thing that they might actually be better at than the Russians is cohesion and discipline,” he said.

Rob Lee, a Russian military specialist at the Foreign Policy Research Institute in Philadelphia who recently returned from a visit to Ukraine, added, “Thousands of additional infantry can make a difference in Kursk. These soldiers are younger and in better physical shape than many Russian contract soldiers.”

In exchange for supplying troops, U.S. officials believe North Korea expects to receive rocket and missile technology from Russia as well as diplomatic support. But Ukrainian officials said North Korea may also be hoping to battle-harden its troops and learn from the tactics being developed in the war.

U.S. defense officials said they did not know if North Korea would send additional reinforcements. A senior Ukrainian official said Ukrainian intelligence officials had predicted that North Korea could send as many as 100,000 troops.

Russia is struggling to meet its monthly recruiting goal of roughly 25,000 troops as its casualties mount, meaning the North Korean soldiers are critical.

Mr. Barros called the North Korean deployment an “alternative pipeline.”

“It is likely not going to be a one-time shipment of 10,000 soldiers,” he said. “It is more likely going to be a way to regularly pull in thousands, perhaps up to 15,000 men a month.”

Andrew E. Kramer contributed reporting from Kyiv.