The rare incursion of two J-11 fighter jets over the "median line" separating the self-ruled island from China drew condemnation from Taipei at the end of March, which called the incident "intentional, reckless and provocative".

Citing remarks by Taiwan Affairs Office spokesman Ma Xiaoguang at a press briefing Wednesday, state-run China Daily said the fly-by was part of a planned routine exercise by the People's Liberation Army (PLA).

"Protecting national sovereignty and territorial integrity has been the sacred duty of the PLA, and it is in the interest of people from both sides of the straits," he said.

China has significantly stepped up diplomatic and military pressure on Taiwan since Beijing-sceptic President Tsai Ing-wen took office in 2016, including staging a series of military exercises near the island.

Dismissing Tsai's vow to forcibly expel Chinese fighter jets if they cross the median line again as "wild talk", Ma said the Taiwanese president's comments showed she was "scared."

"We warn them not to play with fire," he said. "Don't even think about it."

Beijing sees Taiwan as part of its territory awaiting unification, even though the two sides have been ruled separately since the end of a civil war in 1949.

The US diplomatically recognises China over Taiwan but remains the island's chief military ally and arms supplier.

Washington warned China against the use of "force or coercion" on Taiwan after its fighter jet incursion -- which came a week after the US sailed a destroyer and coastguard ship through the Taiwan Strait, angering Beijing.