According to US media reports, the US says China will face consequences if it helps Russia in its invasion of Ukraine.
Russia asked China to provide military assistance after it began its invasion, according to unnamed officials.
The Chinese embassy in Washington did not know about this request.
Ahead of a meeting in Rome on Monday between top US and China officials, there is a warning.
Since the start of the crisis Beijing has expressed strong rhetorical support for Moscow, but has not publicly provided any military or economic support.
According to local media outlets, Russia has recently asked China for military equipment, including drones. The response from China is not known.
Jake Sullivan, US National Security Adviser, said in a CNN interview that there will be consequences for large-scale sanctions evasion efforts or support to Russia.
We will not allow that to happen and allow there to be a lifeline to Russia from these economic sanctions from anywhere in the world.
He said that Beijing may not have understood the full extent of Putin's plan before the invasion.
Mr Sullivan said it was possible that Mr Putin lied to them the same way he lied to Europeans.
The head of the Central Foreign Affairs Commission in China and a member of the Politburo are scheduled to meet with Mr Sullivan in Rome on Monday.
According to a US official quoted by the news agency, Mr Sullivan will tell the Chinese about the consequences of increasing their support for Russia.
The spokesman for China's embassy in Washington DC told US media that he had not heard of Russia's request. The high priority now is to prevent the tense situation from getting out of control.
China has refrained from condemning Russia for the invasion, but has said that Moscow's legitimate security concerns should be taken seriously.
Russian propaganda claims about the war in China have been repeated by Chinese state media and government officials.
Beijing has expressed "unwavering support" for the sovereignty of Ukraine. It is ready to help end the war through diplomacy. China has been urged to do more to stop Russia.
If the EU and US help Ukraine, China helps Russia, that will make the war in Ukraine even more consequential.
The White House decided to make public its claim just as President Biden's top security adviser is going to meet with a Chinese diplomat. It appears to be a tactical move to put pressure on China.
The bigger aim could be to try to make the pros and cons of the relationship between Beijing and Moscow known to the public.
It was just a few weeks ago, as the Winter Olympics opened in Beijing, that Presidents Xi and Putin declared a new alliance that had no limit. Military aid could be part of that.
China condemned the UK, the US and others for giving weapons to the Ukrainian military, saying they were adding fuel to the fire.
If the US intelligence assessment is correct and Beijing follows through, they too would be adding fuel.