"I don't think you can reasonably look at China as impartial in any way," White House National Security Council spokesman John Kirby told reporters.
He noted that China has refrained from criticizing the Russian invasion of Ukraine and has continued to buy Russian oil even as the West piles sanctions on Moscow's energy industry to starve the Kremlin of money to pay for the war.
China, Kirby added, also "keeps parroting the Russian propaganda" to the effect that the US and other countries in the West are to blame for the war for giving such strong support to pro-western Ukraine over the years that Russia felt threatened and justified in invading.
It was the most direct US criticism yet of China's aim to be a middleman in efforts to end the war.
In a summit rich with red carpet pomp, Chinese President Xi Jinping was visiting Russia Tuesday and met with President Vladimir Putin, with the war in Ukraine high on their agenda.
After talks Tuesday they hailed what they called a "new era" in Russian-Chinese relations.
Kirby said the two were linked not so much by an alliance but rather "a marriage of convenience, because that's what I think it is."
China has presented a 12-point position paper on the war which includes a call for dialogue and respect for all countries' territorial sovereignty.
Putin said he was open to talks on Ukraine and praised Beijing's position paper.
Kirby said Russia and China "want to change the rules of that game," meaning the rules based international order.
Still, the United States wants to maintain channels of communication with China, he said.
Kirby added that he is not aware of China having provided military assistance to Russia. The United States has said China is considering this big step, but China denies it.