One Western official said Ukraine’s surprise incursion into Kursk in August had thinned out its forces across the battlefield in the east of the country, leaving them vulnerable to Russian advances. But that official, and US officials, said Ukraine still had a strong defence 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, US officials say. The troops are wearing Russian uniforms and have been equipped by Moscow, but they will probably fight in their own discrete units, US defence officials said.

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

Russia has been training the North Koreans in artillery fire, basic infantry tactics and, critically, trench clearing, US 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 DPRK soldiers could be engaged in combat,” Sabrina Singh, the deputy Pentagon press secretary, said, using the initials of North Korea’s formal name, the Democratic People’s Republic of Korea.

US officials are not sure what constraints the government of Kim Jong-un has put on the use of its forces. However, US 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.

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US 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. US and British military analysts put the current number of Russian troop deaths and injuries at an average of more than 1200 a day.

The North Koreans will be fighting as light infantry, without the benefit of armoured 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 further. It is not clear if the North Korean government will authorise its forces to conduct sustained operations in Ukraine or if they are intended only for the Kursk counter-offensive, according to US defence officials. Some US officials believe that North Korea could order its troops to stop at the border while the Russian forces press deeper into Ukraine.

US 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 despite that inexperience, the North Korean forces are well-organised. “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.”

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In exchange for supplying troops, US 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.

US defence 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.

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.”

This article originally appeared in The New York Times.