Ukrainian President Volodymyr Zelenskyy said Thursday that North Korea is preparing to send around 10,000 soldiers to Russia to support its war against Ukraine, calling the development "a big problem."

Zelenskyy cited Ukrainian intelligence reports indicating that North Korean personnel have already been deployed in Russian-occupied Ukrainian territories, with additional troops being prepared to join the fight.

"We know about 10,000 soldiers of North Korea that they are preparing to send to fight against us," Zelenskyy said during a press conference in Brussels after the European Union summit. "That's really, I think, a big problem."

Zelenskyy suggested that Russia is relying on North Korean forces to compensate for its substantial troop losses, as many young Russians seek to avoid conscription.

"(Russian President Vladimir) Putin is afraid of mobilization very much because social opinion is against his," he said. "That's why he (Putin) is trying to involve other participants of this war."

He emphasized the risk posed by "a second country" helping Russia's war effort, saying he had discussed the matter with US officials and EU leaders during the summit.

Earlier this month, Ukrainian media reported that six North Koreans were among those killed after a Ukrainian missile strike in the partially occupied eastern Donetsk region on Oct. 3. Russia denied using North Korean troops for its war.

If confirmed, it will signal a major development in the military cooperation between Moscow and Pyongyang besides their suspected arms trade, already criticized by the international community.