The United States has confirmed that rounds of Chinese ammunition have been used in battlefields in Ukraine and suspects they were fired by Russian forces, government sources said Friday.

Whether the ammunition was supplied by China remains unclear, the U.S. administration sources said, while adding Washington is poised to take action if it is verified Beijing made the shipments.

Amid a myriad of disagreements between the United States and China, officials have recently said Washington possesses intelligence indicating that Beijing is considering sending arms and ammunition to Russia.

"It's something that we are vigilant about and continuing to watch carefully," a U.S. State Department official told Kyodo News.

The United States has notified some of its partners about the confirmation, according to the sources.

The confirmation came as Chinese President Xi Jinping plans to travel to Moscow for a three-day state visit from Monday. In his first trip to Russia since its invasion of Ukraine began in February last year, Xi is widely expected to reaffirm his close ties with Russian President Vladimir Putin.

In talks with Xi, U.S. President Joe Biden warned him of serious consequences if China provides military support to Russia for its war in Ukraine.

The U.S. government has determined that the ammunition found in Ukraine was produced in China after analyzing its composition and other factors, the sources said.

However, they did not disclose what kind of ammunition was found.

With the war in Ukraine entering its second year, the confirmation could further stoke tensions between the United States and China. Biden said earlier this week that he expects to speak to Xi soon, without elaborating.

China has increasingly tried to present itself as a neutral peace broker and repeatedly denied any intention to supply Russia with arms for the Ukraine war. But it has avoided condemning the Russian invasion.

As fighting in eastern Ukraine intensifies, Western officials have said that Russian troops and the Wagner Group, a mercenary force fighting for the Kremlin, have been struggling with shortages of shells.

The founder of the private military company, Yevgeny Prigozhin, last month accused the Russian military and Defense Ministry of not providing enough ammunition for its fighters.

While the Ukrainian military is also facing shortages, the mercenary force is now trying to divert weapons from its operations in other countries for use in the ongoing war, according to the sources.

U.S. news website Politico reported this week that Chinese companies have sent Russian entities 1,000 assault rifles, drone parts and other equipment with potential military uses.

Customs data showed the shipments were made between June and December last year, according to Politico.

It said the data did not show whether the items were sent to Russia specifically to help its war effort in Ukraine.

"We still have the same level of concern that we did last week about the potential for China to supply lethal weapons and lethal capabilities to Russia," White House national security spokesman John Kirby said in a press briefing on Friday. "We still don't believe they've taken it off the table."