KYIV, Ukraine — The North Korean soldiers fighting for Moscow in Russia’s Kursk region are assigned their own patches of land to assault. Unlike their Russian counterparts, they advance with almost no armored vehicles in support.

When they attack, they do not pause to regroup or retreat, as the Russians often do when they start taking heavy losses, Ukrainian soldiers and U.S. officials say. Instead, they move under heavy fire across fields strewed with mines and will send in a wave of 40 or more troops.

If they seize a position, they do not try to secure it. They leave that to Russian reinforcements, while they drop back and prepare for another assault.

They have also developed singular tactics and habits. When combating a drone, the North Koreans send out one soldier as a lure so others can shoot it down. If they are gravely wounded, they have been instructed to detonate a grenade to avoid being captured alive, holding it under the neck with one hand on the pin as Ukrainian soldiers approach.

Sent to Russia to join with Moscow’s troops in Kursk, the North Koreans essentially operate as a separate fighting force, the Ukrainian soldiers and U.S. officials said — distinct in language, training and military culture.

“It’s partly two different militaries that have never trained or operated together and partly, I think, Russian military culture, which is, shall we say, not highly respectful of the abilities and norms and operations of partner forces,” said Celeste A. Wallander, who until Inauguration Day was the Pentagon’s assistant secretary for international security affairs.

The North Koreans are largely special operations troops trained for surgical strike missions, she said, but the Russians have basically used them as foot soldiers.

Last fall, North Korea sent about 11,000 soldiers to aid Moscow’s forces in the Kursk region of southern Russia, where the Ukrainians captured territory with a surprise invasion last summer. Since their first combat engagement in early December, roughly one-third of the North Korean soldiers have been killed or wounded, Ukrainian and U.S. officials said.

Gen. Oleksandr Syrsky, Ukraine’s top military commander, said this week that North Korean losses continued to climb, estimating that almost half of those sent had been either injured or killed, but he warned that they were “highly motivated, well-trained” and “brave.”

Reinforcements are expected “within the next two months,” according to one senior U.S. defense official.

The New York Times spoke to a dozen Ukrainian soldiers and commanders who are engaged in direct combat with North Korean soldiers, as well as four U.S. defense officials and military analysts, to put together a portrait of how the North Koreans operate on the battlefield. The Times also viewed video of North Korean assaults provided by the Ukrainian military.

The U.S. officials requested anonymity to speak frankly about battlefield details. Ukrainian soldiers and their commanders asked to be identified only by their first names.

With 1.2 million troops, North Korea’s military ranks among the world’s largest standing armies, and its entry into the war was a profound escalation in a war now approaching its fourth year.

Even before it sent troops to Russia, North Korea was a major supporter of Russia’s war effort. It has sent Moscow millions of artillery shells — which now account for about half of the Russian munitions fired daily — and more than 100 short-range ballistic missiles, according to Western and Ukrainian intelligence officials.

The Kremlin has denied deploying North Korean soldiers to the battlefield and is taking steps to hide their involvement, officials said.

Ukrainian President Volodymyr Zelenskyy said that one of the captured soldiers was found to have a military ID in the name of a resident of Tuva, southern Siberia. The fake identity used data from a real Russian citizen, Ukrainian intelligence officials said.

Ukrainian claims about attempts to hide North Korean participation could not be independently verified.

While North Korean soldiers provide additional manpower, the Russians have struggled to integrate them into the battlefield.

The difficulties have ranged from minor issues, like finding uniforms small enough to fit North Korean soldiers, to communication problems that have led at least twice to North Korean and Russian forces clashing directly because of mistaken identity, U.S. officials and Ukrainian soldiers said.

The Russians are taking steps to address the issues, Ukrainian soldiers said, but have yet to form a more cohesive fighting force.

“Now they’ve started composing groups that include a translator or someone who speaks Russian with a radio, but these groups are not very effective,” said Andrii, the Ukrainian commander.

Using video from a drone camera, Andrii described an assault soon after it happened earlier this month, offering a window into North Korean tactics.

Viewed through thermal imaging, the North Korean soldiers stood out as small dark specks on the snow-dusted fields. They walked some 5 miles — with many killed along the way — and were massing in a tree line for an assault on a Ukrainian trench a short distance away.

“There are about 50 of them here,” Andrii said.

Some were wounded, the video showed, but they did not retreat. They waited for reinforcements and then attacked. Assault groups were made up of five to eight soldiers.

The North Koreans take many casualties, Andrii said, but keep sending new units.

“It’s just forward, forward,” he said. “It’s motivation, orders and strict discipline.”

The “shock brigade” tactic of soldiers advancing with little concern for the mayhem that awaits them is heavily featured in North Korean military training and propaganda. Adopted from the Korean War days, the strategy has caused many casualties in a war fought over open and flat lands with drones, according to South Korean intelligence officials. But they said the North would consider those losses a necessary cost of becoming more skilled in modern warfare. “It feels like they specifically came here to die, and they know it themselves,” said Oleksii, a platoon commander.

Ukrainian intelligence officials said two North Korean soldiers captured on Jan. 9 were also providing insights into the deployments in Kursk. And Ukrainian Special Operations Forces have released excerpts from a number of diaries and communications collected from the bodies of North Korean soldiers, which U.S. officials said appeared authentic.