Further 1,500 soldiers are believed to have been sent to Russia, said South Korean parliamentarian Park Sun-Won on Wednesday.
Park sits on the country's intelligence committee where the information was presented.
In total, it's about 3,000 soldiers, since Nis has previously stated that North Korea sent 1,500 special forces to Russia in October.
NATO confirms the information in the intelligence reports.
"If these forces are deployed to Ukraine, it would mean a significant escalation of North Korea's support for Russia's illegal war – and another sign of Russia's significant losses at the front", said NATO's spokesperson Farah Dakhlallah in a written statement.
According to information from Nis chief Cho Tae-Yong, North Korea plans to send a total of 10,000 soldiers to Russia by December.
Ukraine's President Volodymyr Zelenskyj has previously stated that his government had information that 10,000 North Korean soldiers are being prepared to join the Russian forces.
North Korea and Russia have significantly strengthened their cooperation over the past two years.
