Seoul, South Korea - More than 1,000 North Korean soldiers have been killed or wounded in Russia's war with Ukraine, South Korea's Joint Chiefs of Staff said on Monday.

The new figure follows a report by Seoul's spy agency to MPs last week, which said at least 100 North Korean soldiers had been killed since entering combat in December. US officials had previously spoken of "several hundred" casualties.

Since strengthening formal ties with the Kremlin, Pyongyang has sent thousands of troops to reinforce the Russian military, including to the Kursk border region, where Ukrainian forces seized territory earlier this year.

"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 Joint Chiefs of Staff said in a statement.

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"We are particularly interested in the possibility of additional deployments" of North Korean soldiers to aid Russia's war effort, they added.

South Korean intelligence also suggests that the nuclear-armed North is "producing and providing self-destructible drones" to Russia to further assist Moscow in its fight against Ukraine, while also supplying "240mm rocket launchers and 170mm self-propelled artillery" for the Russian army.

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 South Korea's 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".

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North Korea's military was also seen constructing a new fence stretching 25 miles along the border with the South, testing electric barbed-wire fences with what appeared to be goats.

The North's border security reinforcement had been underway "for eight months, with as many as 10,000 soldiers mobilized," a military official told reporters.

The stepped-up security measures aim to "prevent defections by North Korean civilians and soldiers southward," the Joint Chiefs of Staff said in the report.