Ukrainian President Volodymyr Zelensky has said there have been more than 3,000 North Korean casualties in Russia's Kursk region.

North Korean soldiers fighting for Russia were given fake military documents with Russian names and birthplaces, the Ukrainian military said, amid claims from Kyiv that Russia is trying to conceal the presence of foreign fighters on the battlefield.

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

Despite the relatively high casualty rate, Zelensky added that North Korea might send even more resources to Russian dictator Vladimir Putin’s armies.

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

The secretive nature of succession means that no one outside its highest elites know who could replace Kim Jong Un if he resigns or dies in office.

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

"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.

North Korean troops are suffering heavy losses in the fighting in Russia’s Kursk region and are facing logistical difficulties as a result of Ukrainian attacks,

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

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.