China has sent additional fighters into Taiwan's air defense zone. The defense ministry said 15 Chinese aircraft, including 12 fighters, entered the air defense identification zone, and the anti-submarine aircraft flew south through the Bashi Channel between Taiwan and the Philippines, Reuters reported.

The ministry added that the Taiwan Air Force had dispatched planes to intercept and warn the Chinese.

Speaking earlier in the day, Taiwan's Foreign Minister Joseph Wu said the United States is worried about the risk of conflict. 

“From my limited understanding of American decision makers watching developments in this region, they clearly see the danger of the possibility of China launching an attack against Taiwan,” he told reporters at his ministry.

“We are willing to defend ourselves without any questions and we will fight the war if we need to fight the war. And if we need to defend ourselves to the very last day we will defend ourselves to the very last day.”

Beijing said its activities around Taiwan are aimed at protecting China's sovereignty. The United States expressed concern about China's movements and said its commitment to Taiwan is "rock solid."

The US Navy said that the USS John S. McCain, a guided-missile destroyer, made a "routine" transit through the Taiwan Strait on Wednesday.