Ukraine said Monday that its troops killed or wounded at least 30 North Korean soldiers who had been deployed by Russia to its western Kursk region, where Ukraine has seized territory.

Thousands of troops from North Korea have come to reinforce Russian forces, including in the Kursk border region where Russia has been clawing back territory after a surprise offensive from Ukrainian forces this summer.

"On December 14 and 15, army units from the Democratic People's Republic of Korea (DPRK) suffered significant losses near the villages of Plekhovo, Vorozhba, Martynovka in the Kursk region of Russia -- at least 30 soldiers were killed and wounded," Ukraine's military intelligence said.

The units are "being replenished with fresh personnel" from North Korea, which Western officials estimate has sent at least 10,000 soldiers to help Moscow.

Russia and North Korea have boosted their military ties since Moscow's invasion.

Russia has begun deploying "a noticeable number" of North Koreans in assaults to push Ukrainian troops out of the Kursk region, Zelensky said on Saturday.

He said that according to his information, "the Russians include (North Koreans) in combined units and use them in operations in the Kursk region", where Ukraine launched an incursion in August.

Zelensky said he has also heard the North Koreans "may be used in other parts of the front line", and that "losses among this category are also already noticeable".

Russia's defence ministry said last week its troops recaptured some small settlements in the Kursk region.

Last month a Ukrainian army source told AFP that Kyiv controls 800 square kilometres of territory there, down from previous claims it controlled around 1,400 square kilometres.