Russia has concentrated about 50.000 troops, including North Korean forces, in preparation for launching an offensive aimed at taking control of territory held by elite Ukrainian troops in Russia's Kursk region, US and Ukrainian officials confirmed to The New York Times.

A new US assessment concludes that Russia has built up forces without the need to withdraw troops from eastern Ukraine, its top priority on the frontline, allowing Moscow to push on multiple fronts simultaneously.

Russian forces want to take back territory Ukraine seized in Kursk this year. So far, they have attacked the Ukrainian positions in Kursk with rockets and artillery fire, but they have not yet launched a major attack, US officials said.

Ukrainian authorities say that in the coming days they expect an attack in which North Korean forces who are currently training with Russian soldiers in the far western part of Kursk will participate.

Some US military and intelligence officials have become more pessimistic about Ukraine's chances, noting that Russia is relentlessly gaining territory, both in Kursk and in eastern Ukraine. Officials say this is the result of Ukraine's failure to address a critical shortage of human combat potential.

Some Western military experts believe that Ukraine's surprise incursion into Kursk in August thinned its forces on the battlefield in eastern Ukraine, leaving them vulnerable to Russian advances on the ground. But the prevailing opinion in the US is that Ukraine still has strong defenses in Kursk and can hold out, at least for a while.

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 in Kursk, US officials said.

The soldiers are dressed in Russian uniforms and equipped by Moscow, but will likely fight in their own units, US military experts said.

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

The training suggests that at least some of the North Korean forces will be involved in frontal assaults on Ukrainian dug-in defensive positions.

US officials are not sure what restrictions have been placed by the government of President Kim Jong-un on the use of its forces. However, US officials expect them to be directly involved in the fighting.

A Ukrainian official said North Korean forces are divided into two groups, an attack unit and a support unit, which will help secure territory that has been captured by Ukrainian forces.

North Korea has a large military, but unlike Russia, it has not engaged in ground combat for decades. The North Korean forces deployed in Russia, however, are considered the best of the North Korean army and come from the 11th Corps, which is made up of special operations troops.

The Ukrainians have captured hundreds of square kilometers of territory with little resistance, but Russia has so far retaken roughly half of the territory and now appears poised to mount a much larger operation.

US military strategists believe that Ukrainian forces will be difficult to dislodge and that Russian and North Korean forces are likely to suffer heavy losses, similar to those suffered by Russia in eastern Ukraine.

American and British military analysts believe that the current number of dead and wounded Russian soldiers averages more than 1.200 per day. The North Koreans will fight as light infantry, without the advantage of armored vehicles.