South Korea's spy agency said on Friday it had confirmed that a North Korean soldier sent to back Russia's war against Ukraine had been captured by Ukrainian forces.

Pyongyang has deployed thousands of troops to reinforce Russian troops, including in the Kursk border region where Ukraine mounted a shock border incursion in August.

"Through real-time information sharing with an allied country's intelligence agency, it has been confirmed that one injured North Korean soldier has been captured," South Korea's National Intelligence Service said in a statement.

The soldier was captured by the Ukrainian army, an intelligence source told AFP news agency, adding that the location where he was seized was unknown.

The first confirmation of the capture of a North Korean soldier came days after Ukrainian President Volodymyr Zelensky said on Monday that nearly 3,000 North Korean soldiers had been "killed or wounded" so far. Seoul's Joint Chiefs of Staff also said on Monday that more than 1,000 North Korean soldiers have been killed or wounded.

North Korean state media said on Friday that Russian President Vladimir Putin sent a New Year's message to North Korean leader Kim Jong Un, saying "the bilateral ties between our two countries have been elevated after our talks in June in Pyongyang".

A landmark defence pact went into effect in December after the two sides exchanged ratification documents. Putin hailed the deal in June as a "breakthrough document". (AFP)