More than 1,000 North Korean soldiers have been killed or wounded fighting alongside Russian troops in the Ukraine war, South Korea's Joint Chiefs of Staff said on Monday.

The new figure follows a report by Seoul's spy agency to lawmakers last week, which said at least 100 North Korean soldiers had been killed since entering combat in December.

"Through various sources of information and intelligence, we assess that North Korean troops who have recently engaged in combat with Ukrainian forces have suffered around 1,100 casualties," the South Korean military said in a statement.

"We are particularly interested in the possibility of additional deployments" of North Korean soldiers to aid Russia's war effort, it added. Pyongyang is reportedly "preparing for the rotation or additional deployment of soldiers."

According to the South Korean military, intelligence suggests that Pyongyang is "producing and providing self-destructible drones" to Russia to assist Moscow in its fight against Ukraine. The North has also supplied "240mm rocket launchers and 170mm self-propelled artillery," South Korea's Joint Chiefs of Staff said.

Seoul's military noted that North Korea was aiming to modernize its conventional warfare capabilities based on combat experience in the Russia-Ukraine war.

"This could lead to an increase in the North's military threat toward us," it said.

The latest findings align with a report by the National Intelligence Service, which informed lawmakers that "Russia might offer reciprocal benefits" for North Korea's military contributions, including "modernizing North Korea's conventional weaponry."

Pyongyang has sent thousands of troops to reinforce the Russian military, according to Western governments, including to the Kursk border region where Ukrainian forces seized territory earlier this year.