China is considering giving Russia arms and ammunition for the Ukraine war, US Secretary of State Antony Brinken said.
Blinken told CBS News that Chinese companies were already providing "non-lethal support" to Russia, and new information suggested Beijing could provide "lethal support".
This escalation would mean "serious consequences" for China, he warned.
China has denied reports that Moscow has requested military equipment.
Chinese President Xi Jinping, an ally of Russian President Vladimir Putin, has yet to condemn Russia's aggression but seeks to remain neutral in the conflict and seeks peace. China's foreign ministry said it would not accept any "finger pointing" or "coercion" by the United States over its relations with Russia.
Brinken told CBS after meeting with Chinese diplomat Wang Yi at the Munich Security Conference on Saturday.
At the meeting, he said he had expressed “deep concerns" about the "possibility that China will provide lethal material support to Russia".
"To date, we have seen Chinese companies... provide non-lethal support to Russia for use in Ukraine. The concern that we have now is based on information we have that they're considering providing lethal support," he said.
He did not elaborate on intelligence received by the United States about possible Chinese plans. Emphasizing what the US believes China can give Russia, he said it would be primarily weapons and ammunition.
The US has imposed sanctions on a Chinese company for supplying Ukrainian satellite imagery to the mercenary Wagner Group, which supplies thousands of fighter jets to Russia. "of course, in China, there's really no distinction between private companies and the state". Blinken told CBS.
If China were to supply Russia with weapons, it would "serious problem for us and in our relationship", he added.
Relations between Washington and Beijing had already deteriorated after the US shot down an alleged Chinese spy balloon in early February. Both sides exchanged angry words, but equally both sides seemed perplexed by the incident and ready to move on. But US-China relations would deteriorate further if China supplied weapons to support Russian forces in Ukraine.
Mr. Blinken's warnings are clearly aimed at discouraging China from doing so.
Brinken also said the United States was concerned that China could help evade Western sanctions aimed at hurting Russia's economy. China's trade with Russia is growing and it is one of Russia's largest markets for oil, gas, and coal.
NATO member states, including the United States, have sent various weapons, ammunition, and equipment to Ukraine, including tanks. They have stopped sending fighter jets, and Blinken did not say whether the United States would help supply jets to other countries.
"We've been very clear that we shouldn't fixate or focus on any particular weapons system," he said.
But the West said Ukraine must secure what it needs for a possible counterattack against Russia "in the months ahead". Russia is now trying to push into eastern Ukraine, which has seen some of the hardest fightings in the war.
Wang said in Munich on Saturday that China "neither stood by idly nor thrown fuel on the fire" over the war in Ukraine, according to Reuters.
China will release a document outlining its position on dispute resolution, Wang said. The document states that the territorial integrity of all countries must be respected, he said.
"I suggest that everybody starts to think calmly, especially friends in Europe, about what kind of efforts we can make to stop this war," Wang said.
"Some forces that seemingly don't want negotiations to succeed, or for the war to end soon", he added but did not specify what whom he was referring to.
Chinese President Xi Jinping will deliver a "peace speech" on Friday, February 24, to mark Russia's invasion of Ukraine, according to Italian Foreign Minister Antonio Tagjani.
Mr. Tajani told Italian radio that Mr. Xi's speech was a call for peace without blaming Russia, Reuters reported.
During the meeting, Blinken and Wang also exchanged strong words about the growing dispute over a Chinese spy balloon that was allegedly shot down over the United States.
At the meeting, Mr. Blinken said the United States "stand for any violation of our sovereignty" and said, "this irresponsible act must never again occur".
Blinken told CBS that other countries were concerned about China’s “surveillance balloon program" across five continents.
Wang, meanwhile, called the incident a “political farce manufactured by the US" and accused it of "using all means to block and suppress China". China denies sending spy balloons.
And on Sunday morning, Beijing warned that the US "fully accepts all consequences" if the balloon dispute escalates. In a statement reported by Reuters, the State Department said China would "hang out to the end" if "the US insists on taking advantage of the issue"