Taiwan scrambled jet fighters Wednesday after Chinese warplanes buzzed the island for the third straight day, its defence ministry said, the latest in a recent string of incursions sparked by a high-level US visit.

Two Chinese Y-8 anti-submarine aircraft entered Taiwan's southwest air defence identification zone (ADIZ) the ministry said in a statement, adding its jets had broadcast warnings to leave.

Foreign minister Joseph Wu on Tuesday said Beijing "must back off" following the recent incursions that started on September 16, the day before a high-ranking US diplomat was due to visit Taiwan.

Angered by the trip, China twice sent its fighters and bombers into Taiwan's ADIZ and across the "median line" of the Taiwan Strait during undersecretary of state Keith Krach's visit to Taipei.

A Chinese foreign ministry spokesman later said there was no such thing as the median line "as Taiwan is an inseparable part of Chinese territory," sparking condemnations from Taipei.

China considers democratic Taiwan part of its territory, to be absorbed into the mainland, by force if necessary, even though the island has been self-ruled for more than seven decades.

Beijing has ratcheted up pressure on Taiwan since the 2016 election of President Tsai Ing-wen, who rejects its view that Taiwan is part of "one China".

Washington's increased outreach to Taiwan under President Donald Trump has become yet another flashpoint with Beijing, as the US and China clash over a range of trade and security issues, as well as the coronavirus pandemic.