The spy agency estimated around 2,000 North Korean troops dispatched to Russia for combat in Ukraine were killed, lawmakers said Tuesday.

The National Intelligence Service (NIS) shared the assessment during a closed-door briefing to the parliamentary intelligence committee, Reps. Park Sun-won and Lee Seong-kweun told reporters.

The NIS said Pyongyang is planning to additionally send some 6,000 soldiers to Russia in its third batch of troop deployment to aid Moscow's war with Ukraine, while around 1,000 combat engineers have already arrived in Russia.

The existing troops are stationed in the "rear front as reserve forces," the agency said, pointing to the possibility of a leadership change among the dispatched troops.

"North Korea revealed it sustained around 350 deaths in the first and second rounds of deployment, while the NIS reported at least 600 deaths in a briefing to the parliamentary intelligence committee in April," Lee said, noting that the NIS has increased its estimate of the death toll to 2,000 after a "comprehensive review" with other countries.

Since October last year, North Korea has sent around 13,000 troops and conventional weapons to support Russia's war efforts.