Tensions have soared in recent months as Beijing steps up air incursions near Taiwan, a self-ruling democracy that the growing Asian power has vowed one day to take over, by force if necessary.

At a CNN televised forum in Baltimore on Thursday evening, Biden was asked whether the United States would come to Taiwan's defense if China invaded. "Yes," he responded. "We have a commitment to that."

Biden's statement appeared at odds with the long-held US policy of "strategic ambiguity," where Washington helps build Taiwan's defenses but does not explicitly promise to come to the island's help in the event of war.

The White House clarified Friday that it was still guided by the 1979 Taiwan Relations Act, in which Congress required the United States to provide weaponry to "enable Taiwan to maintain sufficient self-defense capabilities."

"The president was not announcing any change in our policy and there is no change in our policy," a White House spokesperson said.

"We will uphold our commitment under the act to support Taiwan's self-defense, and we will continue to oppose any unilateral changes in the status quo," the spokesperson said.

Defense Secretary Lloyd Austin, on a visit to NATO headquarters in Brussels, declined to discuss "hypotheticals" but said the United States "will continue to help Taiwan with the sorts of capabilities that it needs to defend itself."

- New firmness? -

The policy is designed to deter a Chinese invasion but also discourage Taiwan from formally declaring independence -- something Beijing regards as a red line.

Biden's comments were welcomed on Friday by Taiwan, which has pushed to bolster international alliances to protect itself from Beijing.

"The US government has demonstrated, through actual actions, their rock solid support for Taiwan," Presidential Office spokesperson Xavier Chang said in a statement.

But Beijing said that Biden's comments risked "damaging Sino-US relations," warning Washington on Friday to "act and speak cautiously on the Taiwan issue."

"China has no room for compromise on issues involving its core interests," foreign ministry spokesman Wang Wenbin said at a regular press briefing.

The US should not underestimate China's "staunch determination, firm will and strong ability" to defend against what it sees as threats to its sovereignty, Wang added.

Biden made a similar pledge in August during an interview with ABC, insisting that the US would always defend key allies, naming Taiwan among them, despite the withdrawal from Afghanistan in the face of the victorious Taliban.

Biden's remarks were unlikely to be out of ignorance. The veteran politician himself was serving on the Senate Foreign Relations Committee in 1979 and voted to approve the Taiwan Relations Act.

Richard McGregor, a senior fellow for East Asia at the Lowy Institute, said Biden may have wanted to show greater firmness.

"I suspect Biden was not trying to announce any change. So it was either loose language, or perhaps a slightly harder tone, deliberately adopted because of the way Beijing has increased the tempo of its military harassment of Taiwan recently," he told AFP.

- Warplane incursions -

China has ramped up economic, diplomatic and military pressure on Taiwan since the 2016 election of President Tsai Ing-wen, who views Taiwan as already sovereign and not part of "one China."

Military pressure has escalated in the last year with China sending waves of fighter jets and nuclear-capable bombers into Taiwan's air defense zone.

According to an AFP tally, more than 800 flights have been made into the zone since September last year -- 170 just this month.

Defending Taiwan, one of Asia's most progressive democracies, has become a rare bipartisan issue in Washington's otherwise deeply polarised landscape.

At Thursday's live town hall, Biden was also asked by an audience member whether the United States would be able to keep up with China's rapid military development.

Biden responded with "Yes."

"Don't worry about whether... they're going to be more powerful," he said. "China, Russia and the rest of the world knows we have the most powerful military in the history of the world."

Biden's comments come in the wake of a Financial Times report that China has tested a state-of-the-art hypersonic missile with nuclear capacity that flew around the planet before landing, albeit not on target.

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