Relations between self-ruling Taiwan and China have deteriorated since Tsai Ing-wen came to power, largely because she refuses to accept the "One China" formula governing relations.

Beijing regards the island as its territory -- to be reunited by force if necessary -- even though the two sides split in 1949 after a civil war.

China's growing military is increasingly flexing its muscles and will hold live-fire drills next week in the Taiwan Strait -- the narrow waterway separating the Chinese mainland from Taiwan -- following weeks of naval manoeuvres in the area.

Tsai on Friday boarded the Kee Lung destroyer to supervise as Taiwanese sailors practised defending against an attack on the island's northeastern port of Suao.

The exercise was staged in light of a "changing international and regional security situation" to test the military's combat readiness and its ability to defend Taiwanese territory, the island's defence ministry said.

Some 20 warships took part in the exercise, alongside four F16 fighter jets.

Tsai has warned against what she called Beijing's "military expansion" -- the increase in Chinese air and naval drills around the island since she took office in May 2016.

Chinese warplanes conducted 25 drills around Taiwan between August 2016 and mid-December last year, according to Taipei.

On Thursday, Chinese leader Xi Jinping made a surprise visit to naval forces in the disputed South China Sea, where he stressed the "urgent" need to build a powerful navy.

The Liaoning carrier group sailed through the Taiwan Strait on March 20, the same day that Xi issued a public warning against attempts to "separate" from China.

Xi's visit came after a US aircraft carrier gave a demonstration Tuesday for members of the Philippine government.