Unnamed US officials reportedly told multiple news outlets that Russia had asked China to provide military assistance after it began the invasion.
The Chinese foreign ministry accused the US of spreading disinformation. Russia denied asking Beijing for military help.
The exchanges came before top US and Chinese officials met in Rome.
US media outlets, citing Washington officials, say that Russia has in recent days asked China specifically for military equipment, including drones.
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”.