US President Joe Biden has warned China is "flirting with danger" over Taiwan, and vowed to intervene militarily to protect the island if it is attacked.
Speaking in Japan, he appeared to contradict long-standing US policy in the region, although the White House insisted there had been no departure.
Mr Biden drew a parallel between Taiwan and Russia's invasion of Ukraine, prompting an angry rebuke from Beijing.
He is on his first tour of Asia as US president, visiting regional allies.
Mr Biden prefaced his remarks saying US policy toward Taiwan "has not changed". But his comments in Tokyo are the second time in recent months he has unequivocally stated the US would defend Taiwan if China attacked, in what has been seen as a change in tone.
The US has previously been vague on what it would do in such a situation.
China regards Taiwan as a breakaway province that must be re-unified with the mainland.
Beijing's foreign ministry spokesman Wang Wenbin insisted: "Taiwan is an inalienable part of China's territory... there's no room for compromise or concession.
"The Taiwan question and the Ukraine issue are fundamentally different. To compare those two is absurd. We once again urge the US to abide by the One China principle."
The US has no official diplomatic ties with Taiwan, but sells arms to it as part of its Taiwan Relations Act, which states that the US must provide the island with the means to defend itself.
At the same time, it maintains formal ties with China and also diplomatically acknowledges China's position that there is only one Chinese government.
What did Biden say - and why does it matter?
Mr Biden was answering questions in Tokyo during a press conference with Japanese PM Fumio Kishida, when a journalist asked them about the defence of Taiwan.
The US president began by directly linking the China-Taiwan situation to Russia's invasion of Ukraine. If there was a rapprochement eventually between Ukraine and Russia, and sanctions were not sustained, "then what does this signal to China about the cost of attempting to take Taiwan by force?" he asked.
"They are already flirting with danger right now by flying so close and all the manoeuvres that they are undertaking," Mr Biden said, referring to increasing reports of Chinese warplane incursions into Taiwan's self-declared air defence zone.