Beijing quickly rebuffed the claim this week but experts say it may have some merit and, if China did give support, could be a "game changer" in the year-old conflict.
Here are some key questions about Washington's claim, and its implications:
What is behind the US claim?
Since Russian tanks rolled over the border into Ukraine, China has offered Putin diplomatic and financial support, but refrained from overt military involvement or sending caches of lethal arms.
Chinese state-controlled firms have sold non-lethal drones and other equipment to both Russia and Ukraine, but Moscow has been forced to turn to Iran for much-needed supplies such as unmanned aerial vehicles. The United States has said North Korea has also provided rockets and artillery shells.
Washington believes that might be about to change, and on Sunday Secretary of State Antony Blinken made those fears public.
"Based on information we have... they're considering providing lethal support," he said of the Chinese.
Blinken provided no evidence to back up the claim -- and critics will point to past US intelligence failures -- but it follows the pattern of Washington releasing sensitive information to preempt and disrupt Russian war plans.
"The fact that Mr Blinken has chosen to make his concerns public suggests that the US has robust intelligence," said Richard McGregor, a senior fellow for East Asia at the Lowy Institute in Sydney.
Beijing did not comment directly on any closed-door deliberations, but accused Washington of "spreading false information" and "shifting blame".
Why is Washington concerned?
Throughout the war, Russia has struggled to muster enough personnel, munitions and weapons to overpower fierce Ukrainian resistance -- forcing Putin to turn to mass conscription, mercenary groups and imports.
Meanwhile, Ukraine managed to halt the Russian juggernaut and even gain an upper hand. But some experts believe the war is at an inflexion point, with each side clamouring for resources and eying decisive gains as Winter moves into Spring.
Against this backdrop, an influx of Chinese weapons would be "a game-changer", Mick Ryan, a strategist and retired Australian Army major general told AFP.
"This is a war of industrial systems. At the moment Russia is overmatched by the West. If China comes along, any advantage Ukraine had because of the industrial capacity of the West disappears instantly."
Chinese "munitions would make life very difficult for the Ukrainians, whether it's artillery ammunition, whether it's precision munitions or longer-range strike weapons which Russia is running out of".
Why would China get involved?
Chinese military commentator Song Zhongping insisted China would not send arms, but said political, trade and military cooperation between Moscow and Beijing had deepened before the Ukraine war and would continue.
"China will not listen to the United States' demands. China will strengthen cooperation with Russia in accordance with its own national will and national security concerns," he said.
Many experts believe there is a bigger game at hand, and see Ukraine becoming a Cold War-style proxy conflict.
"The war in Ukraine is crunch time for the international security environment, for the world order," said Alexey Muraviev a professor of security and strategic studies at Perth's Curtin University.
A Chinese decision to export weapons would be "a huge step" that risks Western sanctions, burns remaining bridges with Washington and scuttles ties with Europe.
But Muraviev believes the prospect of a Russian defeat is at least as worrying for Beijing.
"If Russia is to lose in Ukraine politically or militarily, China will be left alone," he said. "Russia is the only major power which supports China."
Conversely, a Russian victory would mean "inflicting a strategic defeat on the United States", he said, helping resuscitate President Xi Jinping's narrative that the West is in decline -- an idea damaged by Russia's poor showing in Ukraine.
"For the Chinese, Russia's failure to achieve victory last year was a bit of a cold shower," he said. "They began reassessing their own capacity to run a similar campaign."
"The war in Ukraine shows that you can have lavish military parades, shiny large-scale exercises, but the real test of whether your military is up to the task comes on the battlefield."
Muraviev believes that China may try to thread the needle between risk and reward in Ukraine by supplying weapons via state-controlled companies, North Korea or to the Wagner Group rather than directly to Russian military regulars.
"I think their approach will be more clandestine," he said.
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