South Korea "won't sit idle" over the North's deployment of thousands of troops to help Russia fight Ukraine, President Yoon Suk Yeol said on Thursday, slamming it as a "provocation".

Seoul's spy agency has claimed that about 3,000 North Korean soldiers are currently in Russia training, likely to deploy to the frontlines against Ukraine soon, with thousands more troops to be sent by December.

Nato and Washington have confirmed that Pyongyang's soldiers are now training in Russia, warning that if they were to join the fight against Kyiv, it could mark a dangerous escalation of the grinding war.

"South Korea won't sit idle over this," Yoon said after talks with visiting Polish President Andrzej Duda, adding that the two countries agreed North Korea's deployment was "a provocation that threatens global security beyond the Korean Peninsula and Europe".

Poland's Duda spoke to Yoon on Thursday, after which the two leaders "strongly condemned North Korea's nuclear and missile development and provocations“.

North Korea has previously denied deploying forces to Russia, while Moscow has not confirmed it, but has defended its military cooperation with Pyongyang. (AFP)