Pelosi was the highest-profile US official to head to Taiwan in years, defying stark threats from Beijing, which views the self-ruled island as its territory.

China launched a series of exercises in multiple zones around Taiwan in response, straddling some of the busiest shipping lanes in the world.

Beijing is yet to formally confirm whether missiles overflew the islands during the drills, while Taipei has refused to confirm or deny the flight paths, citing intelligence concerns.

But Japan's defence ministry said of the nine missiles it had detected, four were "believed to have flown over Taiwan's main island".

And on Friday, a hashtag shared by state media asking "what does it mean for the People's Liberation Army's conventional missiles to pass over Taiwan island?" attracted some 43.7 million views on China's Twitter-like Weibo.

"Our exercises this time included live-firing tests, and it was the first time they crossed Taiwan island," Meng Xiangqing, a professor at China's military-affiliated National Defence University, told state broadcaster CCTV, lauding the accuracy of Beijing's capabilities.

He added that they passed through an airspace where Patriot missiles -- a highly mobile surface-to-air missile system that would be a crucial defence against Chinese warplanes -- are densely deployed.

The latest drills also represented the PLA's closest-ever exercises to the island, its first encirclement and the first time it set up a shooting range east of Taiwan, Meng said.

China's official Xinhua news agency reported that the military "flew more than 100 warplanes including fighters and bombers" during the exercises, as well as "over 10 destroyers and frigates."

The latest drills are expected to continue until midday Sunday, and have sparked outrage from the United States, Japan and the European Union, as well as Taipei.

White House spokesman John Kirby called it an overreaction by China and a "pretext" to increase military activity around the Taiwan Strait.

China defends the drills as just countermeasures in the face of provocations by the United States and its allies in Taiwan.