China stepped up a show of force around Taiwan on Wednesday, sending more fighter jets into the island's self-defense zone.
Taiwan's foreign minister responded by warning it would fight to the end in case of war.
The democratic self-governed island, which China claims as its own, has complained of escalating military activity by Beijing in recent months.
On Monday, China said an aircraft carrier group was exercising around the island, while the U.S. Navy said it had conducted a quote "routine" transit of the Taiwan Strait on Wednesday.
U.S. State Department spokesman Ned Price on Wednesday described Washington's commitment to Taiwan as quote "rock solid."
"We have, of course, taken note with great concern the pattern of ongoing PRC efforts and attempts to intimidate in the region, including in the context of Taiwan. In support of longstanding U.S. policy, again, as reflected in the Taiwan Relations Act, the United States maintains the capacity to resist to to resist any resort to force or other forms of coercion that would jeopardize the security or the social or economic system of the people on Taiwan."
Washington is Taiwan's most important international backer and arms supplier, and its been pushing Taipei to modernize its military and become a "porcupine" hard for China to attack.
Taiwan's Foreign Minister Joseph Wu said Taiwan was determined to improve its military capabilities and spend more on defense, and would run war simulations later this month.
China, which claims its Taiwan activities are aimed at protecting Chinese sovereignty, did not respond to requests for comment on Wu's remarks.