North Korean troops dispatched to Russia have moved to the frontline region of Kursk and are "already engaging in combat" against Ukraine, South Korea's spy agency said Wednesday.

"North Korean soldiers dispatched to Russia have moved to the Kursk region over the past two weeks and their deployment to battlefields has been completed," the National Intelligence Service said.

It said work is underway to gather and analyze additional intelligence, without providing further details.

The US has confirmed that most of the more than 10,000 North Korean soldiers sent to Russia have moved to the far western Kursk region, where they have begun engaging in combat against Ukraine.

In Brussels on Wednesday (local time), US Secretary of State Antony Blinken also told reporters at the North Atlantic Treaty Organization that North Korean troops are "quite literally" in combat.

The South Korean spy agency disclosed the information a day after North Korea reported leader Kim Jong-un's ratification of a mutual defense treaty that he signed with Russian President Vladimir Putin in June.

The treaty, which commits the two nations to providing military aid to each other in the event of war, has pushed the countries' relations to new heights.

South Korea has pledged to take necessary measures that will be rolled out in stages according to progress in the military cooperation between North Korea and Russia. (Yonhap)