Ukraine on Wednesday expanded on its claim that significant numbers of Chinese nationals are fighting for Russia’s invading army, saying it had gathered detailed intelligence on more than 150 mercenaries Moscow allegedly recruited through social media. In China, officials called the allegations “totally unfounded.”

The Ukrainian accusation and Chinese denial come as the U.S. strives to secure a ceasefire in the more than three-year war.

President Volodymyr Zelenskyy announced Tuesday that the Ukrainian military had captured two Chinese men fighting alongside the Russian army on Ukrainian soil. It was the first time Ukraine had made such a claim about Chinese fighters in the war. On Wednesday, Zelenskyy said he was willing to exchange the two prisoners of war for Ukrainian soldiers held captive in Russia. Without providing evidence, Zelenskyy said officials in Beijing were aware of Russia’s campaign to recruit Chinese mercenaries. He stopped short of saying the Chinese government authorized the mercenaries’ involvement in Ukraine.