US President Joe Biden is expected to tell Chinese President Xi Jinping on Friday that Beijing will pay a price if it supports Russia's military operations in Ukraine, a warning that comes at a time of deepening conflict between the nations.
Biden and Xi will speak by phone, scheduled for 9 am Eastern time (1300 GMT), and Washington has already threatened lower-level Chinese government officials privately and publicly that greater support for Russia risks isolation for Beijing.
The conversation between Biden and Xi, their first since November, will be loaded with tension.
Biden "will make clear that China will bear responsibility for any actions it takes to support Russia's aggression, and we will not hesitate to impose costs," US Secretary of State Antony Blinken said on Thursday.
"We're concerned that they're considering directly assisting Russia with military equipment to use in Ukraine," he said.
China has denied such plans.
Washington is also concerned that China could help Russia circumvent economic sanctions imposed by Western nations.
Russia's invasion of Ukraine, now in its fourth week, has killed hundreds of civilians, reduced city areas to rubble and sparked a humanitarian crisis as millions flee the country.
It has also added a new front in the contentious US-Chinese relationship, deflating Biden's initial hopes of easing a wide range of disputes by using a personal connection with Xi that predates his term in office.
Biden greeted Xi warmly during the first moments of a video conference in November. Xi called Biden an "old friend."
US officials increasingly view relations with Beijing through the prism of inherent competition, even though they want to avoid a 'cold war' or a direct confrontation between the rival powers.
Washington sees China growing even closer to Russia after Xi and Russian President Vladimir Putin last month announced a "no-limits" strategic partnership last month.
China has refused to condemn Russia's action in Ukraine or call it an invasion, and it has censored online content in China that is pro-West or unfavorable to Russia.
Beijing, while saying it recognizes Ukraine sovereignty, has also said Russia has legitimate security concerns that should be addressed, and has urged a diplomatic solution to the conflict.
While Biden's administration has threatened unspecified counter-measures if China helps Russia's effort in Ukraine, officials have grown more resigned to that likelihood.
The United States and its allies have not yet decided on precisely what steps to take against China, according to one person involved in those conversations.
Targeting Beijing with the sort of extensive economic sanctions imposed on Russia would have potentially dire consequences for the United States and the world, given that China is the second largest economy and largest exporter.
A seven-hour meeting in Rome on Monday between lower-level aides to Biden and Xi were described as "tough" and "intense" by officials.
Biden's administration has not yet offered evidence of the claim that China has signaled a willingness to help Russia.
Moscow has denied asking China for military assistance, and China's foreign ministry called the idea "disinformation."
Russian Finance Minister Anton Siluanov said this week the country was counting on China to help it withstand the blow to its economy from punishing Western sanctions aimed at isolating Russia's economy from the rest of the world.
The European war is not the only challenging topic on the agenda for Biden and Xi.
The two leaders are also expected to compare notes on the Iran nuclear talks, North Korea's missile launches and Taiwan. China and the United States are also engaged in separate trade talks.
(This story has been refiled to correct day to Thursday in fourth paragraph)