Ukrainian President Volodymyr Zelenskiy said North Korea is preparing to send 10,000 troops to help Russia's war on Ukraine, in a sign of deepening military cooperation between Moscow and Pyongyang.

"We know about 10,000 soldiers of North Korea, that they are preparing to send to fight against us," Zelenskiy told reporters in Brussels on Thursday. Ukraine officials have for weeks said that North Korean soldiers were fighting in Ukraine, without providing evidence.