US House of Representatives Speaker Nancy Pelosi left Taiwan on Wednesday after pledging solidarity and hailing its democracy, leaving a trail of Chinese anger over her brief visit to the self-ruled island that Beijing claims as its own.

China demonstrated its outrage over the highest-level US visit to the island in 25 years with a burst of military activity in surrounding waters, summoning the US ambassador in Beijing and halting several agricultural imports from Taiwan.

Some of China's planned military exercises were to take place within Taiwan's 12 nautical mile sea and air territory, according to Taiwan's defence ministry, an unprecedented move a senior defence official described to reporters as "amounting to a sea and air blockade of Taiwan".

Pelosi arrived with a congressional delegation on her unannounced but closely watched visit late on Tuesday, defying China's repeated warnings, in what she said shows unwavering US commitment to Taiwan's democracy.

"Our delegation came to Taiwan to make unequivocally clear that we will not abandon Taiwan,” Pelosi told Taiwan President Tsai Ing-wen, who Beijing suspects of pushing for formal independence — a red line for China.

“Now, more than ever, America’s solidarity with Taiwan is crucial, and that’s the message we are bringing here, today," she said during her roughly 19-hour visit.

“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”.

A long-time China critic, especially on human rights, Pelosi met with a former Tiananmen activist, a Hong Kong bookseller who had been detained by China, and a Taiwanese activist recently released by China.

Fury on the mainland over the 82-year-old Democrat's defiance of Beijing was evident all over Chinese social media, with one blogger railing: "This old she-devil, she actually dares to come!"

Read: China says US will ‘bear all consequences’ if Pelosi visits Taiwan

The last US house speaker to go to Taiwan was Newt Gingrich, in 1997. But Pelosi's visit comes amid sharply deteriorating Sino-US relations, and during the past quarter century, China has emerged as a far more powerful economic, military and geopolitical force.

China considers Taiwan part of its territory and has never renounced using force to bring it under its control. The United States warned China against using the visit as a pretext for military action against Taiwan.

In retaliation, China's customs department announced a suspension of imports of citrus fruits, chilled white striped hairtail and frozen horse mackerel from Taiwan, while its commerce ministry banned the export of natural sand to Taiwan.

While there was little sign of protest against US targets or consumer goods, there was a significant police presence outside the US consulate in Shanghai and what appeared to be more security than usual outside the embassy in Beijing.

Military drills

Shortly after Pelosi's arrival, China's military announced joint air and sea drills near Taiwan and test launches of conventional missiles in the sea east of the island, with Chinese state news agency Xinhua describing live-fire drills and other exercises around Taiwan from Thursday to Sunday.

China's foreign ministry said Pelosi's visit seriously damages peace and stability in the Taiwan Strait, "has a severe impact on the political foundation of China-US relations, and seriously infringes upon China's sovereignty and territorial integrity".

Editorial: Taiwan tensions

Before Pelosi's arrival, Chinese warplanes buzzed the line dividing the Taiwan Strait. The Chinese military said it was on high alert and would launch "targeted military operations" in response to Pelosi's visit.

White House national security spokesperson John Kirby said after Pelosi's arrival in Taiwan that the United States "is not going to be intimidated" by China's threats or bellicose rhetoric and that there is no reason her visit should precipitate a crisis or conflict.

Kirby said China might engage in "economic coercion" toward Taiwan, adding that the impact on US-China relations will depend on Beijing's actions in the coming days and weeks.

US Secretary of State Antony Blinken discussed the potential for Pelosi's visit with counterpart Wang Yi during a G20 meeting in Bali last month and said any such trip would be entirely Pelosi's decision and independent of the US government, a senior US official said on Wednesday.

'China's ambition'

The United States has no official diplomatic relations with Taiwan but is bound by American law to provide it with the means to defend itself. China views visits by US officials to Taiwan as sending an encouraging signal to the pro-independence camp on the island. Taiwan rejects China's sovereignty claims, saying only the Taiwanese people can decide the island's future.

Taiwan's military increased its alertness level. Its defence ministry said 21 Chinese aircraft entered its air defence identification zone on Tuesday, and that China was attempting to threaten key ports and cities with drills in the surrounding waters.

"The so-called drill areas are falling within the busiest international channels in the Indo-Pacific region," a senior Taiwan official familiar with its security planning told Reuters on Wednesday.

"We can see China's ambition: to make the Taiwan Strait non-international waters, as well as making the entire area west of the first island chain in the western pacific its sphere of influence," the person said.

China's foreign ministry said it has not seen its military drills around Taiwan causing any freedom of navigation issues.

Pakistan reaffirms support for 'One China' policy

Meanwhile, Pakistan reaffirmed its strong commitment to "One China" policy, under which it is acknowledged that there is only one Chinese government and the US recognises formal ties with China rather than Taiwan.

In a statement issued by the Foreign Office in Islamabad on Wednesday, Pakistan said it firmly supported China’s sovereignty and territorial integrity while expressing deep concern over the evolving situation in the Taiwan Strait.

It said the situation "has serious implications for regional peace and stability".

"The world is already reeling through a critical security situation due to the Ukraine conflict, with destabilising implications for international food and energy security. The world cannot afford another crisis that has negative consequences for global peace, security and economy," the statement read.

It added: "Pakistan strongly believes that inter-state relations should be based on mutual respect, non-interference in internal affairs, and peaceful resolution of issues by upholding of principles of UN charter, international law and bilateral agreements".

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.

Additional input from AFP.