The US President, Joe Biden, said his country "would intervene" to defend Taiwan if it were attacked by China.
In some of his strongest comments to date, the President told CBS News "if in fact, there was an unprecedented attack," US troops would defend the island.
In response, Chinese Foreign Ministry spokesperson Mao Ning said he was making "solemn complaints" to the US over the comments.
"China strongly deplores and rejects it," he said. "The US remarks seriously violate the one-China principle."
Taiwan is an island off the coast of China which is a self-governing democracy.
However China refuses to recognise the island as independent and claims it as part of its territory.
While the US has stopped short of recognising Taiwan as an independent country, strong informal ties exist between the two, and the US sells Taiwan weapons under the Taiwan relations act.
However, it officially adheres to the 'one-China policy', which means it only has formal diplomatic relations with Beijing.
Nancy Pelosi, the Speaker of the US House of Representatives, visited Taiwan at the beginning of August, sparking huge Chinese military drills off the coast of the island.
Four days ago the US senate approved a massive $6.5 billion pound bill to send weapons to Taiwan.