More than 1,000 North Korean soldiers have been killed or wounded in Russia's war with Ukraine, South Korea said Monday, while Kyiv put the total at over 3,000.
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.
South Korea's Joint Chiefs of Staff (JCS) said in a statement that "we assess that North Korean troops who have recently engaged in combat with Ukrainian forces have suffered around 1,100 casualties," citing "various sources of information and intelligence".
It gave its figure following 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.
Ukrainian President Volodymyr Zelensky later gave a far higher estimate, writing on X that "according to preliminary data, the number of North Korean soldiers killed and wounded in the Kursk region has already exceeded 3,000".
Zelensky warned of "risks of North Korea sending additional troops and military equipment".
Pyongyang is reportedly "preparing for the rotation or additional deployment of soldiers" to aid Russia's war effort, the JCS said.
Seoul said its 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.
The North was also supplying "240mm rocket launchers and 170mm self-propelled artillery" for the Russian army, the JCS said.
Seoul's military noted that North Korea was aiming to modernise 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 Seoul's National Intelligence Service, which informed lawmakers that "Russia might offer reciprocal benefits" for North Korea's military contributions, including "modernising North Korea's conventional weaponry".
- 'Destabilisation risk' -
North Korea and Russia have strengthened their military ties since Moscow's invasion of Ukraine in February 2022.
A landmark defence pact between Pyongyang and Moscow, signed in June, came into force this month.
Experts say North Korean leader Kim Jong Un is keen to acquire advanced technology from Russia and battle experience for his troops.
Pyongyang lashed out on Thursday at what it called "reckless provocation" by the United States and its allies for a joint statement criticising North Korea's support for Russia's war in Ukraine, including the deployment of troops.
Zelensky warned Monday that growing cooperation between Moscow and Pyongyang caused a proportionately increased "risk of destabilisation around the Korean Peninsula".
South Korea and Ukraine announced last month that they would deepen security cooperation in response to the "threat" posed by the deployment of North Korean troops, but there was no mention of potential arms shipments from Seoul to Kyiv.
South Korea's suspended President Yoon Suk Yeol said in November that Seoul was "not ruling out the possibility of providing weapons" to Ukraine, which would mark a significant shift in its long-standing policy barring the sale of weapons to countries in active conflict.
North Korea's military was also seen constructing a new fence stretching 40 kilometres (25 miles) along the border with the South, testing electric barbed-wire fences with what appeared to be goats.
A photo provided by the JCS shows a North Korean soldier holding what appears to be a goat in front of barbed-wire fences.
The North's border security reinforcement had been under way "for eight months with as many as 10,000 soldiers mobilised", a military official told reporters.
The stepped-up security measures aim to "prevent defections by North Korean civilians and soldiers southward", the JCS said in the report.
The North has also launched around 7,000 trash-carrying balloons into the South on 32 occasions since May, Seoul's military said.
Activist groups in South Korea have long sent propaganda northwards, typically carried by balloons, including leaflets, US dollar bills and sometimes USB drives containing K-pop or K-dramas, which are banned in the tightly controlled North.
Pyongyang rails at such activity and has said its trash-carrying balloon offensive is in retaliation for the activists' propaganda efforts.
While Pyongyang has refrained from launching such balloons since November 29, "indications of their readiness for a surprise launch at multiple sites" have been observed, Seoul's military said.