Ukraine on Wednesday expanded on its claim that significant numbers of Chinese nationals are fighting for Russia's army, saying its detailed intelligence showed more than 150 mercenaries Moscow allegedly recruited through social media.

But in China, officials called the allegations "totally unfounded."

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

President Volodymyr Zelenskyy announced Tuesday that his military captured two Chinese men fighting alongside the Russian army on Ukraine 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.

Zelenskyy said Ukraine has the last names and passport data for 155 Chinese citizens fighting for the Russian army and that "we believe that there are many more of them." He shared with journalists documents listing names, passport numbers and personal details of the alleged Chinese recruits, including when they arrived in Russia for military training and departed for service; the AP has not independently verified the documents.

China has provided strong diplomatic support for Russia since it launched its invasion of Ukraine in February 2022. It also has allegedly sold Russia machinery and microelectronics that it can use to make weapons, according to Western officials, in addition to providing an economic lifeline through the trade in energy and consumer goods.

China is not believed to have knowingly provided Russia with troops, weapons or military expertise.

U.S. officials have accused Iran of providing Russia with drones, and U.S. and South Korean officials said North Korea has sent thousands of troops and ammunition to help Russia.

State Department spokesperson Tammy Bruce said at a briefing Tuesday that reports of Chinese citizens fighting on behalf of Russia were "disturbing."

"China is a major enabler of Russia in the war in Ukraine," Bruce said, adding that China provides nearly 80% of the dual-use items Russia needs to sustain the war.

The European Union's foreign policy chief, Kaja Kallas, also called Beijing "the key enabler of Russia's war."

Dual-use goods are entering Russia via China, she said in Brussels, adding "it's clear that if China would want to really stop the support then it would have an impact."

China has surged sales to Russia of machine tools, microelectronics and other technology that Moscow is using to produce missiles, tanks, aircraft and other weaponry for the war, according to a U.S. assessment last year.

The Kremlin has effectively rejected a U.S. proposal for an immediate and full 30-day halt in the fighting in Ukraine. The Kyiv government has consented to it. Both sides are believed to be readying spring-summer military campaigns.

Chinese Foreign Ministry spokesperson Lin Jian said China has played a "constructive role in politically resolving the Ukraine crisis."

Lin told a daily news briefing Wednesday that "the Chinese government always asks Chinese citizens to stay away from conflict zones, avoid getting involved in any form of armed conflict, and especially refrain from participating in any party's military operations."

His comments appeared to indicate that the captured Chinese had joined Russia's ranks on their own. Russia and Ukraine allow foreign soldiers to enlist.

China has previously put forward a vague peace plan that was swiftly dismissed by most observers.