© EPAMr Sullivan, seen here in a file picture, warned that the US would not allow China to provide a "lifeline" to Russia
Unnamed US officials reportedly told multiple US news outlets that Russia had asked China to provide military assistance after it began the invasion.
The Chinese foreign ministry did not address the allegation directly, but accused the US of maliciously spreading disinformation about China.
The exchanges come as top US and Chinese officials are to meet in Rome.
US media outlets, citing Washington officials, say that Russia has in recent days asked China specifically for military equipment, including drones.
Comment: Russia and China are trade partners, it's likely they've been in talks about a variety of things, including drones and military equipment.
In a CNN interview, US National Security Adviser Jake Sullivan said they were "communicating directly, privately to Beijing that there will absolutely be consequences for large-scale sanctions evasion efforts or support to Russia to backfill them".
"We will not allow that to go forward and allow there to be a lifeline to Russia from these economic sanctions from any country, anywhere in the world," he said.
Comment: In many ways, it's all hot air, but the phrasing is revealing: the US does not want Russia to have a 'lifeline'.
He added that while the US believed China was aware that Russian leader Vladimir Putin was "planning something" before the invasion happened, Beijing "may not have understood the full extent of it".
Comment: Maybe not, but China was probably aware that Ukraine was planning an attack on Donbass, and that it wanted to get the materials together to create a dirty bomb, and so China probably had a good idea of how critical it was for Russia to act, and that it would have to be decisive.
Bear in mind that China has already faced the West's hybrid warfare in Hong Kong, and it's looking like it will face a similar challenge in Taiwan (known officially as the People's Republic of China), and so it's probable that it too will be forced to make equally tough decisions in the future.
"Because it's very possible that [Mr] Putin lied to them the same way that he lied to Europeans and others," Mr Sullivan said.
In response, a spokesman for the foreign ministry in Beijing, Zhao Lijian, said the US was spreading disinformation.
He added that China's stance had always been consistent and that China was playing a constructive role in promoting talks.
Comment: Quite unlike Ukraine's nomination for a mediator, apartheid-state Israel, that has been supplying the neo-Nazi military in Ukraine with weaponry for many years, and who receives billions in aid from the US.
Mr Sullivan is due to meet Yang Jiechi, a member of China's top decision-making body, the Politburo, and the head of the Central Foreign Affairs Commission, on Monday in Rome.
Reuters news agency quoted a US official as saying that during the meeting, Mr Sullivan would spell out the consequences and isolation China would face if it increased support for Russia.
China has so far refrained from condemning Russia for the invasion and has said Moscow's "legitimate security concerns" should be taken seriously.
When the United Nations General Assembly voted to condemn Russia's invasion earlier this month, China was one of 35 countries that abstained.
Comment: Does the US intend on sanction all of those 35 countries? Or those that choose to do business with them? Could it be that they abstained because they can see Russia had little choice but to act? It's unlikely they will, because even their allies, like those in the UAE have ignored their threats, and for others the US has had to make exceptions. Meanwhile the EU have admitted that they don't intend on sanctioning Russian gas, just yet, because they have no other options.
But Beijing at the same time has expressed "unwavering support" for Ukraine's sovereignty.
Comment: China is well aware that Ukraine lost its sovereignty following the US-backed coup in 2014.
It has also called for peace and has said it is ready to help end the war through diplomacy. Several countries have urged China to do more to stop Russia's invasion.
The EU and US help Ukraine, China helps Russia; if that's how this goes, then it's a delineation that will make the war in Ukraine an even more consequential one.
The White House has decided to make public its claim just as President Biden's top security adviser is due to meet China's most senior diplomat. It appears to be a tactical move, to put pressure on China; presumably to either confirm or deny it.
The bigger aim could be to try to make Xi Jinping weigh up the pros and cons to his current position of what was last week called a "rock solid" relationship with Moscow.
Comment: Again, even US allies are ignoring Washington, it's laughable to think that China would compromise its relations with Russia for an untrustworthy, ailing hegemon like the US, particularly because it is also a victim of US belligerence.
Remember that it was just 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, clearly, be part of that.
But in the days after Russia's invasion China has condemned the UK, the US and others for giving weapons to Ukraine's military, saying they were adding "fuel to the fire".
If the US intelligence assessment is correct and Beijing follows through on that request, then they too would be "adding fuel".
There, fixed it.