Volodymyr Zelenskyy says two Chinese soldiers have been captured while fighting in eastern Ukraine.
He said his forces had fought six Chinese soldiers and two of them had been taken prisoner. He added that he had ordered officials to obtain an explanation from Beijing.
“We have information that there are many more Chinese citizens in the occupier’s units than just two. We are now finding out all the facts,” he added.
Follow latest: Zelenskyy demands reaction from US
China is an ally of Russia and has been accused of helping its war in Ukraine, though Beijing has repeatedly denied allegations that it has supplied Kremlin forces with weapons.
“Russia’s involvement of China in this war in Europe, directly or indirectly, is a clear signal that Putin is going to do anything but end the war,” Mr Zelenskyy said. “He is looking for ways to continue the war.”
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Zelenskyy makes dig at US over response to Russian attack
Mr Zelenskyy described China as having joined Russia’s war against Ukraine and said he expected the US to react.
Fighters of various nationalities have joined Russia’s army during the war, often in return for promises of large sums of money.
North Korea has also sent thousands of its own troops to support Russia.
Last night, Mr Zelenskyy confirmed for the first time that Ukrainian forces are active in Russia’s Belgorod region.
He described the actions as “just”, adding: “war must return to where it came from”.