North Korea and Russia are deepening their military cooperation, as Pyongyang ramps up the supply of arms to Moscow for its war in Ukraine and receives much needed cash and oil from the Kremlin in return.

North Korean troops have now entered Russia’s war with Ukraine in a major way, and some have been killed, Western officials say.

Despite their elite status, North Korea's "Storm" troops were ill-prepared for the war, South Korea's National Intelligence Service said.

South Korea's military has said that North Korea is preparing to continue aiding Russia in its war with Ukraine, despite casualties.

There are risks of North Korea sending additional troops and military equipment to the Russian army,” Zelensky said. “We will have tangible responses to this.”

South Korea's military said on Monday it has detected signs of North Korea preparing to send more troops and weapons, including suicide drones, to Russia to support its war against Ukraine.

In Trump-led armistice talks, the Hermit Kingdom may want a seat at the table.

"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 JCS said in a statement.

Ukrainian President Volodymyr Zelensky confirmed Monday more than 3,000 troops from North Korea have been killed so far fighting next to Russian forces.

Secretive North Korea has one of the world’s largest militaries, with 1.28 million active soldiers, but unlike in Russia, the Korean People's Army (KPA) has no recent experience of combat. Pyongyang’s army is “thoroughly indoctrinated but with low readiness,” says Mark Cancian, from the Centre for Strategic and International Studies (CSIS).

Ukrainian President Volodymyr Zelenskiy said on Monday that more than 3,000 North Korean soldiers have been killed and wounded in Russia's Kursk region and warned that Pyongyang could send more personnel and equipment for Moscow's army.