Twenty-seven Chinese warplanes flew into Taiwan's air defence zone on Wednesday, Taipei said, as US House Speaker Nancy Pelosi made her controversial visit to the self-ruled island that Beijing considers its territory.

"27 PLA aircraft... entered the surrounding area of (Republic of China) on August 3, 2022," the defence ministry said in a tweet.

Taiwan maintained a defiant tone as it hosted Pelosi, with a furious China gearing up for military exercises dangerously close to the island's shores in retaliation for the visit.

Pelosi landed in Taiwan on Tuesday despite a series of increasingly stark threats from Beijing, which views the island as its territory and had said it would consider the visit a major provocation.

China responded swiftly, announcing what it said were "necessary and just" military drills in the seas just off Taiwan's coast -- some of the world's busiest waterways.

"In the current struggle surrounding Pelosi's Taiwan visit, the United States are the provocateurs, China is the victim," Beijing's foreign ministry said.

But Taiwanese President Tsai Ing-wen said the island of 23 million would not be cowed.

"Facing deliberately heightened military threats, Taiwan will not back down. We will... continue to hold the line of defence for democracy," Tsai said at an event with Pelosi in Taipei.

She also thanked the 82-year-old US lawmaker for "taking concrete actions to show your staunch support for Taiwan at this critical moment".

China tries to keep Taiwan isolated on the world stage and opposes countries having official exchanges with Taipei.

Pelosi, second in line to the presidency, is the highest-profile elected US official to visit Taiwan in 25 years.

"Today, our delegation... came to Taiwan to make unequivocally clear we will not abandon our commitment to Taiwan," she said at the event with Tsai.

She added her group had come "in friendship to Taiwan" and "in peace to the region".

Before leaving Taiwan, Pelosi also met with several dissidents who have previously been in the crosshairs of China's wrath -- including Tiananmen protest student leader Wu'er Kaixi.

"We are in high agreement that Taiwan is in the frontline (of democracy)," Wu'er said.

"Both the United States and Taiwan governments need to... conduct more in defending human rights."

Pelosi's delegation left Taiwan on Wednesday evening headed to South Korea, her next stop in an Asia tour. She will head to Japan after.

 'High alert'

The administration of President Joe Biden said in the run-up to the visit that US policy towards Taiwan remained unchanged.

This means support for its government while diplomatically recognising Beijing over Taipei, and opposing a formal independence declaration by Taiwan or a forceful takeover by China.

While the White House is understood to be opposed to Pelosi's Taiwan stop, its National Security Council spokesman John Kirby said she was entitled to go where she pleased.

Beijing summoned US Ambassador Nicholas Burns over Pelosi's visit, while the Chinese military declared it was on "high alert" and would "launch a series of targeted military actions in response" to the visit.

The drills will include "long-range live ammunition shooting" in the Taiwan Strait, which separates the island from mainland China and straddles vital shipping lanes.

The zone of Chinese exercises will be within 20 kilometres (12 miles) of Taiwan's shoreline at some points, according to coordinates released by the Chinese military.

"Some of the areas of China's drills breach into... (Taiwan's) territorial waters," defence ministry spokesman Sun Li-fang said at a press conference Wednesday.

"This is an irrational move to challenge the international order."

Taiwan's Mainland Affairs Council, which sets the government's China policies, accused Beijing of "vicious intimidation" that would "seriously impact the peace and prosperity of the entire East Asia".

It added that democratic countries should "unite and take a solemn stand to punish and deter" Beijing.

Japan, a key US ally in the region, said Wednesday it had expressed concern to China over the exercises, while South Korea called for dialogue to maintain regional peace.

- 'We shouldn't be too worried -
Beijing has long used diplomatic, military and economic pressure on Taiwan.

On Wednesday China announced curbs on the import of fruit and fish from Taiwan -- citing the detection of pesticide residue and the coronavirus. It also halted shipments of sand to the island.

"Those who offend China will be punished," Chinese Foreign Minister Wang Yi told reporters Thursday while on a trip to Cambodia.

Outside the Taiwanese parliament, 31-year-old computer programmer Frank Chen shrugged off the Chinese warnings against Pelosi's visit.

"I'm not too worried about China's intimidation," he told AFP.

"I think China will take more threatening actions and ban more Taiwanese products, but we shouldn't be too worried."

There was a small group of pro-China demonstrators outside parliament as well.

"The United States uses Taiwan as a pawn in its confrontation with China, to try to drag China down so (it) can dominate the world," Lee Kai-dee, a 71-year-old retired researcher, told AFP.

"If the United States continues to act this way, Taiwan will end up like Ukraine."

China has vowed to annex self-ruled, democratic Taiwan one day, by force if necessary.

Russia's invasion of Ukraine in February heightened fears in Taiwan that China may similarly follow through on its threats to annex the island.