Taiwan's Central News Agency said the two F-18s landed at an air force base in southern Taiwan on Wednesday after experiencing mechanical problems. It said it was not clear where they were coming from or where they were going.

"We have already made solemn representations to the U.S. side," China's Foreign Ministry spokeswoman, Hua Chunying, told a regular news briefing.

"China demands that the United States strictly abide by the 'one-China policy' ... and cautiously and appropriately handle this incident."

The United States is obligated to help Taiwan defend itself, under the Taiwan Relations Act of 1979, when Washington severed formal ties with the island to recognize the People's Republic of China in Beijing.

U.S. weapons sales in recent years to Taiwan, or indeed any formal contact between the two armed forces, have provoked strong condemnation by China, but have not caused lasting damage to Beijing's relations with either Washington or Taipei.

China views Taiwan as a renegade province and has not ruled out the use of force to bring it under its control.

While Taiwan and China have signed a series of landmark trade and economic agreements since 2008, political and military suspicions still run deep, especially in democratic Taiwan, where many fear China's true intentions.

China's military modernization has also been accompanied by a more assertive posture in its regional territorial disputes.