Taiwan struck a defiant tone Wednesday as it hosted US House Speaker Nancy 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, warning the US ambassador in Beijing of "extremely serious consequences" and announcing military drills in seas around Taiwan -- some of the world's busiest waterways.

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

She also thanked Pelosi 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.

Earlier, Pelosi said her group had come "in friendship to Taiwan" and "in peace to the region".

The Taiwan visit of US House Speaker Nancy Pelosi (C) has sent tensions soaring between Washington and Beijing
The Taiwan visit of US House Speaker Nancy Pelosi (C) has sent tensions soaring between Washington and Beijing AFP / Sam Yeh

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.

After Pelosi touched down Tuesday night in a military aircraft following days of feverish speculation about her plans, the Chinese foreign ministry summoned US Ambassador Nicholas Burns.

Her visit "is extremely egregious in nature and the consequences are extremely serious", Vice Foreign Minister Xie Feng told Burns, according to state news agency Xinhua.

"China will not sit idly by."

The Chinese military said 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.

Map of Taiwan and its surrounding waters, highlighting the areas of the Chinese military drills from August 4 to 7.
Map of Taiwan and its surrounding waters, highlighting the areas of the Chinese military drills from August 4 to 7. AFP / Laurence CHU

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 and stability.

Both countries are on Pelosi's Asia itinerary, following stops in Singapore, Malaysia and Taiwan.

China on Wednesday 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.

Chinese Foreign Minister Wang Yi vowed Thursday to punish those who offend Beijing.

"This is a complete farce. The United States is violating China's sovereignty under the guise of so-called 'democracy'... those who offend China will be punished," he said.

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.

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.