"Beijing will have to make its own decisions about how it proceeds, whether it provides military assistance - but if it goes down that road it will come at real costs to China," Sullivan told CNN's "State of the Union" program.
The United States and its NATO allies in recent days have been scrambling to dissuade China from such a move, making public comments on their belief that China is considering providing lethal equipment to Russia.
U.S. Secretary of State Antony Blinken on Friday said China has been providing nonlethal assistance to Russia through its companies.
Putin has alluded to a Xi visit but the timing has not been confirmed by Russia or China.
Russia and China signed a "no limits" partnership in February 2022 shortly before Russian forces invaded Ukraine.