China reacted angrily on Thursday to a US military transport jet's flight over the self-ruled island of Taiwan, calling it an "illegal act" and "serious provocation".

The overflight adds to rising tensions between Washington and Beijing over a slew of issues, and the US relationship with the island claimed by China is high on the list of disagreements.

Taiwan's defence ministry said a Boeing C-40A Clipper transport jet flew over the island on Tuesday, the same day Taiwan intercepted several Chinese fighter jets that flew into the island's southwest airspace.

A spokeswoman for Beijing's Taiwan Affairs Office said the US flight "harms our sovereignty, security and development interests, and violates basic principles of international law and international relations."

The Taiwan Affairs Office is Beijing's top-level body overseeing policy towards the self-governing democratic island, which China considers its own territory to be reunited with the mainland by force if necessary. 

Zhu said Taiwan's ruling Democratic Progressive Party (DPP) "linked up with external forces to violate China's territorial sovereignty, destroy the peace and stability of the Taiwan Strait, and harm the safety and well-being of islanders." 

Taiwan's defence ministry said the US aircraft entered the island's airspace after applying for permission and did not land at any of its airports.

Cross-strait relations have plunged to new depths after Taiwan's independence-leaning President Tsai Ing-wen was re-elected by a landslide in January on an anti-reunification platform.

Since then, Taiwan has bought arms from France and the US -- its key ally -- to counter growing military threats from China.

China has ramped up fighter flights and warship crossings near Taiwan or through the Taiwan Strait since Tsai was first elected in 2016, as she has refused to acknowledge that the island is part of "one China".

Tsai lashed out at Beijing for "meaningless and unnecessary" military moves after a Chinese military jet in February briefly crossed the median line of the Taiwan Strait separating the two sides.

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