China protested on Thursday after two American jet fighters landed at an air base in Taiwan, which Beijing regards as its own territory, reportedly for the first time in 30 years.

Two US F-18 fighter jets made an emergency landing at an air force base in the southern city of Tainan on Wednesday, with US authorities saying one of the planes had developed a mechanical failure.

"We have launched solemn representations with the US," Hua Chunying, a foreign ministry spokeswoman, told a regular briefing in Beijing.

"We require the US to abide by the 'One-China Policy' and the three joint communiques between China and the US and to prudently deal with the relevant issue," she added, referring to agreements between the two that recognise Beijing as the sole government of China.

Taiwanese media described the landing as the first of its kind since the mid-1980s and speculated that it could have been a US reaction to an unprecedented People's Liberation Army Air Force exercise over the western Pacific Ocean east of Taiwan.

Several Chinese aircraft on Monday flew over the ocean for the first time via the Bashi Channel, which runs between Taiwan and the Philippines, Beijing's official Xinhua news agency said.

Mark Zimmer, spokesman for the American Institute in Taiwan (AIT) -- the de facto US embassy -- said the planes had been on a "routine flight" when one encountered a mechanical problem and requested an emergency landing.

The incident "again highlights the close ties between Taiwan and the United States," according to Taiwanese MP and military expert Lin Yu-fang of the ruling Kuomintang party, who sits on the parliament's diplomacy and defence committee.

The two planes were flying over waters east of Taiwan escorting an EA-6B Prowler, an electronic warfare aircraft, on its way to the Philippines, he said, citing government sources.

- 'A trusted place' -

"Taiwan must have been considered by the United States a trusted place to make the emergency landing," he told AFP.

The US does not have formal diplomatic ties with Taiwan, officially known as the Republic of China, but has strong economic links with it and is a key military supplier.

Beijing regularly proclaims the importance of its "One China" policy, seeing Taiwan as part of its territory awaiting reunification, and often curtails the island's involvement in international agreements.

The two split in 1949 at the end of the Chinese civil war, with defeated Nationalist forces retreating to the island as Mao Zedong's victorious Communists set up the People's Republic.

Kevin Cheng, editor in chief of the Taipei-based Asia-Pacific Defense Magazine, described the planes' landing as "an unexpected incident" -- but said such incidents would probably become more common.

"Things like this are expected to be on the rise in the future as the United States has been intensifying its military presence in Asia while China is becoming ever more active militarily," he said.

"One of the potential flash points is the South China Sea, where China is reclaiming more land on the islets it administers. The United States has been concerned about the development and is beefing up military cooperation with the Philippines."

The White House is pursuing its foreign policy "pivot" towards Asia, seen by many as an attempt by Washington to counter China's meteoric rise and increasing influence in the area.

The move has alarmed China, which has been ramping up its military budget and expanding its capabilities. While Beijing says its actions are purely defensive, it has raised concerns among its neighbours and heightened territorial tensions.

"Whether intended or not, this 'accident'... reminds China that the United States maintains a robust 'unofficial' military relationship with Taiwan," Rick Fisher, an expert on Chinese military affairs at the US-based think tank International Assessment and Strategy Center, told AFP.

"Today that relationship does not include overt military cooperation, but could transition to such very quickly if China attacks Taiwan."