China's Foreign Ministry expressed anger on Thursday after two U.S. fighter jets landed in Taiwan, in a rare official contact between the militaries of the United States and the self-ruled democratic island.

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.

"While this landing was unplanned and occurred exclusively out of mechanical necessity, it reflects well on Taiwan that they permitted pilots in distress to land safely," said U.S. Pentagon spokeswoman Henrietta Levin.

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

"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.

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.