South Korea's spy agency said Friday, December 27, 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, 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 Monday that nearly 3,000 North Korean soldiers had been "killed or wounded" so far.

Seoul's Joint Chiefs of Staff (JCS) also said Monday that more than 1,000 North Korean soldiers have been killed or wounded.

The JCS had also said that Pyongyang is reportedly "preparing for the rotation or additional deployment of soldiers" and supplying "240mm rocket launchers and 170mm self-propelled artillery" to the Russian army.

Seoul's military believes that North Korea was seeking to modernise its conventional warfare capabilities through combat experience gained in the Russia-Ukraine war.

North Korean state media said 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."