About 1,500 North Korean soldiers, including those from the special forces, have already arrived in Russia, according to South Korea’s spy agency.
Pyongyang has not commented on the allegations.
South Korea has long accused the North of supplying weapons to Russia for use in the war against Ukraine, but it says the current situation has gone beyond the transfer of military materials.
Some South Korean media reports have suggested as many as 12,000 North Korean soldiers are expected to be deployed, the BBC said.
