The United States stands by its One China policy that sees Taiwan as a part of China, but Washington would intervene if Beijing was to use force against the island, Joe Biden said while speaking at a news conference in Tokyo on Monday.
When asked if the US would use military force, the US president replied: "Yes."
"That's the commitment we made."
A Chinese attack against the self-ruled island would "just not be appropriate," Biden said. "It will dislocate the entire region and be another action similar to what happened in Ukraine."
He said the burden to protect Taiwan was "even stronger" after the Russian invasion of Ukraine.
Biden is on a five-day Asia tour. After three days in South Korea, he arrived in Tokyo on Monday morning for the last leg of his trip. His statements appear to clash with the US tradition of "strategic ambiguity," as Washington usually avoids making any such explicit guarantees to Taiwan.
Later on Monday, however, Defense Secretary Lloyd Austin said there had been no policy shift on Taiwan.
"As the president said, our 'one China' policy has not changed," Austin said at a Pentagon briefing. He said Biden had merely stressed the White House's commitment under the Taiwan Relations Act "to help provide Taiwan the means to defend itself."
China says US is 'playing with fire' over an 'internal affair'
But China hit back, saying the US is "playing with fire," the Chinese State Council's Taiwan Affairs Office said.
Washington is "using the 'Taiwan card' to contain China, and will itself get burned," said Zhu Fenglian, a spokeswoman for the council, which is often viewed as China's cabinet.
State news agency Xinhua said Zhu "urged the United States to stop any remarks or actions" that violate previously established principles between the two countries.
Earlier on Monday, Beijing warned Biden not to "underestimate resolve" on the issue of Taiwan, which China considers part of its own territory.
"The Taiwan issue is a purely internal affair for China," said Foreign Ministry spokesman Wang Wenbin. "On issues touching on China's core interests of sovereignty and territorial integrity, China has no room for compromise or concession."
US policy toward Taiwan
The 1979 Taiwan Relations Act does not require the US to step in militarily to defend Taiwan in case of a Chinese invasion. However the policy requires the US to ensure Taiwan has the resources to defend itself and to prevent any unilateral change of status in Taiwan by Beijing.
A White House official told the Reuters news agency that the US policy toward Taiwan remain unchanged.
"He [Biden] reiterated our One China Policy and our commitment to peace and stability across the Taiwan Strait. He also reiterated our commitment under the Taiwan Relations Act to provide Taiwan with the military means to defend itself," the official said.
Under the "One China" policy, the US recognizes Beijing's influence, and does not have official diplomatic ties with Taiwan. However, it does maintain unofficial contact with Taiwan, and has a de facto embassy in the capital, Taipei.
The US also supplies military equipment to Taiwan.