President Joe Biden on Thursday said the United States would defend Taiwan if the island were attacked by China, which considers it part of its territory.

"Yes," he responded when asked in a CNN town hall about defending Taiwan. "We have a commitment to that."

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

He made a similar pledge in August during an interview with ABC, insisting that the United States would always defend key allies, including Taiwan, despite the withdrawal from Afghanistan in the face of the victorious Taliban.

Biden said the United States made a "sacred commitment" to defend NATO allies in Canada and Europe and it's the "same with Japan, same with South Korea, same with Taiwan."

The White House subsequently told reporters that US policy on Taiwan "has not changed."

Asked by an audience member at the live televised town hall whether the United States would be able to keep up with China's rapid military development, Biden also said, "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."

However, Biden expressed concern that rival countries may "engage in activities where they may make a serious mistake."

He referred to his longtime relationship with Chinese President Xi Jinping and repeated his position that he does not want "to start a new Cold War with China."

But he warned, "I just want to make China understand that we are not going to step back."

China has upped its sabre rattling around Taiwan, sending waves of fighter jets and nuclear-capable bombers crossing into Taiwan's air defense zone.

Biden's comments also 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.

The United States and Russia are racing to develop their own hypersonic weapons, which are more difficult to defend against than existing ballistic missile arsenals.