"As of now, we have information that at least several hundred Chinese nationals are fighting as part of Russia's occupation forces," Zelensky told a gathering of military chiefs from allied countries in Brussels.

"This means Russia is clearly trying to prolong the war -- even by using Chinese lives," he added.

Kyiv said this week that its forces had captured two Chinese nationals in the eastern Donetsk region fighting for Moscow, prompting allegations from Zelensky that Russia was implicating Beijing in the invasion.

The Kremlin denied the claim, while Beijing warned parties to the conflict against making "irresponsible remarks".

Zelensky also issued a plea for more air defence systems, speaking via video from his hometown of Kryvyi Rig where a Russian ballistic missile last week killed 19 people, including nine children.

"You know that Ukraine has a shortage of air defence systems. You know that Patriot systems can effectively protect against ballistic threats," he said.

"I ask you to focus first of all on air defence for Ukraine. We truly need it. Ten Patriot systems -- the free world has them," he added.

The Ukrainian leader also called out Russia for having refused a complete ceasefire proposed by the United States with Ukrainian approval one month ago.

And he called on allies to make progress on creating a contingent of forces that could act as a deterrent against future Russian attacks after a halt to the fighting.

"We need to define clear details regarding the size, structure, deployment, logistics, sustainment, and the equipment and weapons of this security contingent in Ukraine," Zelensky said.