Washington has raised concerns about Beijing helping Moscow in the midst of the bloody but stalled invasion.

"We have not seen... the provision of military equipment by China to Russia. But of course, this is something we are monitoring closely," US National Security Advisor Jake Sullivan said.

President Joe Biden held a nearly two-hour call with his Chinese counterpart Xi Jinping last Friday in which the White House says he warned Xi against bailing out Russia's economy from Western sanctions or assisting Moscow's war effort.

"We will continue to monitor it, and the president made clear to President Xi the implications and consequences of any such provision of equipment, and they very well understand one another," Sullivan added.

Biden is preparing for a trip to Europe where he meets EU leaders and huddles with NATO allies on Thursday, and he "will certainly consult on the question of China's potential participation in the conflict of Ukraine while he's in Brussels," Sullivan said.

Biden and European partners are "very much on the same page" on the issue of China's possible cooperation with Russia, according to the White House.

During Biden's visit, he and European leaders on Thursday will unveil "a further package of sanctions" against Russia and tighten the punitive measures already in place, Sullivan said.

The announcement will not focus only new steps, "but on ensuring that there is joint effort to crack down on evasion on sanctions," he added.