The soldiers are believed to have been sent to Russia in January and February. It is estimated that around 11,000 North Korean soldiers have been sent to fight on Russia's side.
It is implied that North Korea is also preparing to send additional soldiers to Russia, says a military source to Yonhap.
Another military source says that the soldiers' whereabouts are unknown, as the South Korean military cannot track them once they have left North Korea.
But to the news agency AFP, an anonymous military official says that North Korean soldiers have been sent back to the front line in the Russian Kursk region.
The North Korean military was sent back again, after a pause of about a month, to the Kursk front lines, says the official.
Earlier in February, South Korea reported that the North Korean soldiers who had previously fought on Russia's side at the Kursk front had not been engaged in combat since mid-January. Ukraine also reported that the North Korean soldiers had been withdrawn after heavy losses.
A spokesperson for South Korea's defense department did not want to confirm the reports of more soldiers for Yonhap, but emphasized that the department is closely monitoring North Korean military activities.
