Russia has amassed 50,000 soldiers, including North Korean troops, for an offensive to retake the territory seized by Ukraine in Russia's Kursk Region.
The New York Times reports this with reference to US and Ukrainian officials.
The new US assessment says that Russia has amassed forces without having to withdraw soldiers from eastern Ukraine, its top military priority. This allows Moscow to apply pressure on several fronts simultaneously.
The Russians have attacked Ukrainian positions with missile strikes and artillery fire, but they have not yet launched a major offensive there, US officials said. Ukrainian officials say they expect such an attack involving North Korean troops in the coming days.
The North Koreans are currently training with Russian forces in the far western part of Kursk Region.
One Western official noted that Ukraine's surprise invasion of Kursk in August had reduced its forces on the battlefield in eastern Ukraine, leaving them vulnerable to a Russian offensive. But this official and U.S. officials also believe that Ukraine still has strong defences in Kursk and may be able to hold on, at least for a while.
According to U.S. officials, North Korea has sent more than 10,000 soldiers to fight Russia in Kursk. The troops are wearing Russian uniforms and have been equipped by Moscow, but are likely to fight in their own separate units, US defence officials said.
Ukrainian officials said Moscow had provided the North Korean forces with machine guns, sniper rifles, anti-tank missiles and rocket-propelled grenades.
Russia is training the North Koreans in artillery fire, basic infantry tactics and, most importantly, trench clearing, U.S. officials said. This training suggests that at least some of the North Korean troops will engage in frontal attacks on Ukraine's fortified defensive positions.
"We expect that DPRK soldiers may be involved in combat," said Deputy Pentagon spokeswoman Sabrina Singh.