The US is "deeply concerned" about actions that undermine peace across the Taiwan Strait, US national security adviser Jake Sullivan has told the BBC.
His comments come after China sent a "record number" of military jets into Taiwan's air defence zone for four days in a row, in a public show of force.
Taiwan considers itself a sovereign state - but China views the island as a breakaway province.
Beijing has not ruled out the possible use of force to achieve unification.
"We are going to stand up and speak out, both privately and publicly when we see the kinds of activities that are fundamentally destabilising," Mr Sullivan told the BBC's diplomatic correspondent James Landale in Brussels on Thursday, a day after meeting China's top diplomat Yang Jiechi.
Asked whether the US was prepared to take military action to defend Taiwan, Mr Sullivan said: "Let me just say this, we are going to take action now to try to prevent that day from ever coming to pass."
