Taiwan said it scrambled fighters and deployed anti-missile systems Monday after Chinese military jets entered its air defence zone for the fourth time in five days.

Two Chinese Y-8 anti-submarine planes intruded into Taiwan's southwest air defence identification zone (ADIZ), the defence ministry said.

Last week, Chinese fighters and bombers breached the same zone three times -- twice while a high-ranking US diplomat was on a rare trip to the self-ruled island that sparked anger in Beijing.

Washington has sent two envoys to Taipei in as many months, infuriating Beijing, which considers the self-ruled island as part of its territory, to be absorbed into the mainland -- by force if necessary.

Taipei's defence ministry said that Taiwan maintains "the right to self-defence and counter attack" in the wake of the "frequent harassments and threats."

"Our primary principle is not to provoke, not to heighten conflicts, trigger incidents or cause misfire," defence minister Yen De-fa told reporters.

"But we are not afraid of a war and we must maintain our necessary right to self-defence and counter attack."

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

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