Ukraine's top commander says that Russia's military seems to be preparing a major offensive aimed at retaking the territory in the Russian western region of Kursk controlled by Ukrainian forces.

Oleksandr Syrskyi said in his social media post on Monday that the Russians are trying to push out Ukrainian troops and advance deeper into territory under Ukrainian control.

There is a heightened sense of alert with speculation swirling that Russian and North Korean troops will begin a major assault in the region.

The New York Times on Sunday quoted US and Ukrainian officials on the situation. They said that the Russian military has assembled a force of 50,000 soldiers, including North Korean troops, as it prepares to begin an assault aimed at reclaiming territory seized by Ukraine.

The Ukrainian government has recently said about 11,000 North Korean soldiers are present there.

The paper said Ukrainian officials expect such an attack, which would involve the North Koreans, in the coming days.

The paper also said that Russia has reclaimed roughly half the territory seized in Kursk, and now appears ready to conduct a much larger operation there than it has before.