Russia has amassed a large force of tens of thousands — including recently arrived North Korean troops — to carry out an assault on the Ukrainian positions in Russia’s Kursk region expected in the coming days, a US official told CNN on Sunday.

Ukrainian President Volodymyr Zelensky said Thursday that about 11,000 North Korean soldiers are in the region, where Ukraine’s three-month military incursion into Russian territory has stalled.

The New York Times reported Sunday that some 50,000 Russia and North Korean troops are set to take part in the assault.

CNN has reached out to the Ukrainian government for comment.

A Ukrainian commander told CNN Sunday that North Korean troops were taking part in direct combat operations in Kursk, as well as defensive operations in the neighboring Belgorod region of Russia and in Russian-occupied Ukrainian territories.