Taiwan lives under the constant threat of invasion by Beijing, which sees the self-ruled democratic island as part of its territory to be retaken one day, by force if necessary.

The final quarter of 2021 saw a spike in incursions by China into Taiwan's air defence identification zone (ADIZ), with the biggest single-day breach on October 4 when 56 warplanes entered the zone.

On Friday, 18 Chinese aircraft, including 12 J-11 and J-16 fighter jets as well as two H-6 bombers, crossed into the ADIZ, Taipei's defence ministry said.

It was the second-largest single-day sortie this year, after 39 warplanes entered the zone on January 23, according to a database compiled by AFP.

The ministry said it scrambled its own aircraft to broadcast warnings and deployed air defence missile systems to track the jets.

Taiwan only started regularly publicising its data on air incursions in September 2020.

October 2021 remains the busiest month on record, with 196 incursions, 149 of which were made over just four days as Beijing marked its annual National Day.

China has ramped up pressure on Taiwan since Tsai Ing-wen was elected president in 2016, as she considers the island a sovereign nation and not part of Chinese territory.

Last year, Taiwan recorded 969 incursions by Chinese warplanes into its ADIZ, according to the AFP database -- more than double the roughly 380 carried out in 2020.

The figure so far for this year already exceeds 370 as of Friday.

The ADIZ is not the same as Taiwan's territorial airspace and includes a far greater area that overlaps with part of China's own ADIZ.