President Joe Biden said the US would defend Taiwan if China attacked, in an apparent departure from a long-held US foreign policy position.
But a White House spokesman later told some US media outlets that his remarks did not signify a change in policy.
The US has a law which requires it to help Taiwan defend itself.
But it has been deliberately vague about what it would actually do if China were to attack Taiwan, in what is known as "strategic ambiguity".
China has yet to respond to Mr Biden's comments.
At a CNN town hall event, a participant referred to recent reports that China had tested a hypersonic missile. He asked Mr Biden if he could "vow to protect Taiwan", and what he would do to keep up with China's military development.
Mr Biden responded: "Yes and yes." He added there was no need to "worry about whether they're going to be more powerful", because "China, Russia and the rest of the world knows we're the most powerful military in the history of the world".
He was then queried a second time by CNN anchor Anderson Cooper if the US would come to Taiwan's defence in the event of an attack by China. Mr Biden replied: "Yes, we have a commitment to do that."
A White House spokesperson later appeared to walk back Mr Biden's comments, telling US media outlets that the US was "not announcing any change in our policy and there is no change in our policy".
This is not the first time this has happened. In August, Mr Biden appeared to suggest the same stance on Taiwan in an interview with ABC News. The White House had also said then that US policy on Taiwan had not changed.
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.
It has formal ties with China, and also diplomatically acknowledges China's position that there is only one Chinese government.
How have Taiwan and China responded?
Taiwan's presidential office has said it would neither give in to pressure nor "rashly advance" when it gets support.
"Taiwan will show a firm determination to defend itself," said presidential spokesperson Xavier Cheng, who also went on to acknowledge the Biden administration's continued show of "rock-solid" support for Taiwan.
China has not yet responded. But earlier on Thursday, before Mr Biden's town hall, China's UN ambassador Zhang Jun accused the US of "taking dangerous actions, leading the situation in the Taiwan Strait into a dangerous direction".
Tensions have been rising between Taiwan and China in recent weeks after Beijing flew dozens of warplanes into Taiwan's air defence zone.