Western officials told the BBC that North Korean forces have already suffered nearly 40% of casualties in the fighting in the Kursk region, western Russia, in just three months.
Of the estimated 11,000 soldiers sent from North Korea, known as the Democratic People's Republic of Korea, 4,000 died in combat, said the officials, who spoke on condition of anonymity.
This term includes the dead, wounded, missing and prisoners. Of the 4,000, about 1,000 were believed to have been killed by mid-January, officials said.
These losses, if confirmed, cannot be borne by the North Koreans.
It is not clear where the wounded are being treated, nor even when and to what extent they will be replaced.
But the figures suggest a very high cost to an ally of President Vladimir Putin, such as Kim Jong Un, as he seeks to help him expel Ukrainian forces from Russia ahead of any potential ceasefire negotiations later in the year.
Ukraine launched a lightning attack on Russia's Kursk region last August, surprising Russian border guards.
The government in Kiev made clear at the time that it did not intend to keep the territories it had seized, but merely to use them as a bargaining chip in future peace negotiations.
Since then, Ukraine's early gains in Kursk have steadily declined, partly due to the North Koreans' arrival in Russia in October.
But Ukraine still holds several hundred square kilometers of Russian territory and inflicts heavy losses on its enemy.
The North Korean troops, said to belong to an “elite” unit called the Storm Corps, appear to have been thrown into combat with relatively little training or protection.
“These forces are barely trained, and are led by Russian officers who don’t understand them,” says former British Army tank commander Colonel Hamish de Bretton-Gordon.
“Quite frankly, they have no chance. They have been thrown into a meat grinder with little chance of survival. They are cannon fodder, and the Russian officers care less about them than they do their own men.”
Reports attributed to South Korean intelligence say the North Koreans are unprepared for the realities of modern warfare, and appear particularly vulnerable to being targeted by Ukrainian first-person perspective (FPV) drones, a weapon that has been a familiar part of the battlefield. South in the Donbass region of Ukraine for years now.
Despite this, Ukraine's top military commander, General Oleksandr Sersky, warned earlier this week that North Korean soldiers pose a major problem for Ukrainian fighters on the front line.
“They are many. There are 11,000 to 12,000 additional highly motivated and well-equipped soldiers who are conducting offensive actions. They are operating based on Soviet tactics. They are operating in platoons and companies. They are relying on their numbers,” the general told Ukrainian network TSN Tyzhden. News program.
