Taiwan on Tuesday demanded that China "back off" and accused it of threatening peace, after a Beijing official rejected a largely respected marine boundary following recent incursions.

Foreign minister Joseph Wu urged Beijing to "return to the civilised international standards" after a Chinese foreign ministry spokesman said there was no so-called median line in the Taiwan Strait "as Taiwan is an inseparable part of Chinese territory".

Wu told reporters: "The median line has been a symbol of preventing military conflicts and maintaining peace and stability in the Taiwan Strait for many years. The Chinese foreign ministry's comment is equivalent of destroying the status quo."

"I call on the international community to condemn the CCP for its dangerous and provocative words and deeds threatening peace... China must back off," he added in a tweet.

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

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

Last year, Taiwan accused China of violating a long-held tacit agreement after its fighter jets -- for the first time in years -- crossed the median line of the waters that separate the two sides.

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.

Last week, Chinese fighters and bombers twice breached Taiwan's air defence identification zone (ADIZ) while a high-ranking US diplomat was on a rare trip to Taiwan that sparked anger in Beijing.

Taipei's defence ministry said it scrambled fighters again on Tuesday after two Chinese Y-8 anti-submarines planes entered its southwest ADIZ, the fifth such incursion in six days.