TOKYO—President Biden said the U.S. would respond militarily to defend Taiwan if China tries to take it by force, sparking uncertainty over whether the U.S. was moving away from its longstanding policy of strategic ambiguity as the White House looks for ways to check Beijing’s growing clout in Asia.

Mr. Biden’s comments were met with anger from Beijing and praise from Taipei. They were also part of a pattern: In August and October of last year, the president answered questions on Taiwan by suggesting a break in U.S. policy toward the democratically self-ruled island, only to have aides jump in to say nothing had changed.

This time, he chose a venue much closer to Beijing, and the comments came as the president and leaders from a dozen countries in the Indo-Pacific region endorsed a new economic platform meant to counter China’s influence.

Mr. Biden spoke Monday alongside the Japanese prime minister in Tokyo during his first trip to Asia as commander-in-chief.

The president was asked if the U.S. would get involved militarily in response to a Chinese invasion of Taiwan after declining to send American troops to Ukraine to fight Russia’s invasion.

“Yes. That’s the commitment we made,” he said.

A White House official later said that U.S. policy regarding Taiwan hadn’t changed and that Mr. Biden “reiterated our commitment under the Taiwan Relations Act to provide Taiwan with the military means to defend itself.”

President Biden in Tokyo on Monday said he doesn’t expect China will try to take Taiwan by force.

Photo:

Evan Vucci/Associated Press

Under that act, passed in 1979, Congress is committed to selling defensive weapons to Taiwan, but Washington has previously avoided saying whether it would intervene directly in the event of an invasion.

The president’s remarks fueled already heightened tension between Washington and Beijing, which sees Taiwan as a part of China and has vowed to take control of the island.

Chinese Foreign Ministry spokesman

Wang Wenbin

raised his voice when asked at a regular briefing about Mr. Biden’s remarks and said Beijing was strongly dissatisfied by them.

China “has no room for compromise and concession” on core concerns like Taiwan and “will take firm action to safeguard its sovereignty and security interests,” Mr. Wang said.

Mr. Biden has said competition with China is his top foreign-policy objective and has worked to rally allies in applying pressure on economic matters and human rights.

The U.S. has also criticized Beijing’s warming ties with Russia, and Moscow’s invasion of Ukraine has forced U.S. officials to grapple with the prospect of countering two major adversaries at once.

Taiwan is a global flashpoint akin to Ukraine, but with the potential to set off an even more destructive conflict. While there are no signs that war is imminent, China has stepped up military activity in the region in response to what it calls Taiwan independence provocations—and Beijing is expanding its nuclear arsenal.

Mr. Biden, in his Monday remarks, stressed that the U.S. remains committed to the bedrock “One China policy,” which recognizes the present rulers as the only legitimate government and acknowledges—but doesn’t endorse—Beijing’s claim that Taiwan is a part of the nation. But the president said that policy doesn’t give China the right to forcefully take over the island.

“We agree with the One China policy and all the attendant agreements we made. But the idea that it can be taken by force, just taken by force, would just not be appropriate,” Mr. Biden said. “It would dislocate the entire region and be another action similar to what happened in Ukraine. So, it’s a burden that is even stronger.”

He also played down the possibility that China would try to take Taiwan.

“My expectation is that it will not happen, it will not be attempted,” Mr. Biden said, adding that it is important for world leaders to send a strong message that there will be consequences if Beijing takes such action.

Taiwan is thankful to the U.S. for its “rock solid” commitment, Foreign Ministry spokeswoman

Joanne Ou

said.

President Biden began his first trip to South Korea as president with a visit to a Samsung semiconductor factory that will serve as a model for a planned chip plant in Texas, as the U.S. seeks to strengthen supply chains amid competition with China. Photo: Jonathan Ernst/Reuters

“Our government’s determination to firmly defend Taiwan’s freedom, democracy and security has never changed, and we will continue to improve self-defense capabilities,” she said.

Some U.S. lawmakers said Mr. Biden should stick with a firm pledge to defend Taiwan from an attack by China.

“It is essential that President Biden declare the United States will come to Taiwan’s defense in the event of a Chinese attack,” tweeted Sen. Tom Cotton (R., Ark.). “Continued ambiguity will provoke the Chinese communists without deterring them—the worst of both worlds.”

“President Biden is right. Credible deterrence requires both courage and clarity – and Taiwan’s vibrant democracy deserves our full support,” tweeted Sen.

Bob Menendez

(D., N.J.), chairman of the Senate Foreign Relations Committee.

Other Republicans, skeptical of an expansive U.S. role in overseas conflicts, were more cautious. “Biden says he wants to stop Chinese imperialism in Asia – even as he commits tens of THOUSANDS more troops to Europe, indefinitely. Which is it? Must choose a priority,” tweeted Sen.

Josh Hawley

(R., Mo.). He was one of 11 Republicans to vote against the Ukraine aid package last week.

Mr. Biden’s Asia trip was intended to show that Washington is still focused on countering China when Russia’s invasion of Ukraine is getting more public attention. His suggestion of a stiffer line on Taiwan, even if played down by aides, supported his stance that the U.S. isn’t distracted by the Ukraine war and is boosting Asian alliances.

The Indo-Pacific Economic Framework also announced on Monday marks the Biden administration’s most ambitious attempt to build economic ties with Asian nations after the U.S. under then-President

Donald Trump

pulled out of the Trans-Pacific Partnership in 2017. U.S. officials said the framework represented a new approach to cooperation that moves beyond a traditional trade agreement.

The framework will focus on cooperation on global issues such as supply chains, clean energy and digital rules—but it doesn’t include plans to negotiate lower tariffs or broad steps to clear away barriers to market access.

U.S. ally Japan, which long shied away from military commitments in the region, more recently has been at the leading edge of a more assertive posture on Taiwan, making the location of Mr. Biden’s comments significant. Standing next to the president, Prime Minister

Fumio Kishida

said in response to a question on Taiwan that he would work with the U.S. to ensure no one in East Asia could imitate what Russia did to Ukraine.

“We will dramatically strengthen our military,” Mr. Kishida said.

A key backer of the Kishida government, former Prime Minister

Shinzo Abe,

in April called for the U.S. to abandon strategic ambiguity on Taiwan. South Korean President

Yoon Suk

-yeol and Mr. Biden named Taiwan as a security concern after meeting Saturday, a reference the two countries had long refrained from including in joint statements.

Mr. Biden has committed the U.S. to protecting Taiwan in the event of an invasion on previous occasions. In an August interview with ABC News about the U.S. withdrawal from Afghanistan, Mr. Biden included Taiwan on a list of allies he said the U.S. was duty-bound to defend.

At a CNN Town Hall event in October, he responded to a question about the U.S. defending Taiwan by saying, “Yes, we have a commitment to do that.” In both instances, White House officials later said U.S. policy on Taiwan hadn’t changed.

Su Tzu-yun, a researcher at the Institute for National Defense and Security Research, which is backed by the Taiwanese military, said Mr. Biden’s repeated comments and statements about Taiwan by U.S. allies marked a shift.

“The U.S. is moving toward ‘constructive clarity’ from its so-called strategic ambiguity,” Mr. Su said. He called Mr. Biden’s remarks critical to deterring China’s military ambitions.

—Alex Leary, Lindsay Wise and Joyu Wang contributed to this article.

Write to Andrew Restuccia at andrew.restuccia@wsj.com, Ken Thomas at ken.thomas@wsj.com and Josh Chin at josh.chin@wsj.com

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