House Speaker Nancy Pelosi arrived in Taiwan on Tuesday, making her the highest-level US official to visit the island in 25 years, while China's defence ministry announced it would be conducting "targeted military operations".
Ms Pelosi landed in a US military aircraft at 10.44pm local time at Songshan Airport in Taipei.
"Our congressional delegation’s visit to Taiwan honours America’s unwavering commitment to supporting Taiwan’s vibrant democracy," Ms Pelosi said in a statement. She added that her visit does not undermine Washington's "One China" policy.
Ms Pelosi said her visit to Taiwan will focus on promoting shared interests, including support for a free and open Indo-Pacific region. She is expected to meet the president of the territory, Tsai Ing-wen, and visit parliament on Wednesday, Taiwan's central news agency reported.
Pointing to Russia's invasion of Ukraine, the House speaker said it is vital that the US stand by its allies.
"Indeed, we take this trip at a time when the world faces a choice between autocracy and democracy," she wrote in an op-ed for The Washington Post that was published after she landed in Taiwan.
China condemned Ms Pelosi's trip, saying it undermines peace and stability in the Taiwan Strait.
"It has a severe impact on the political foundation of China-US relations and seriously infringes upon China's sovereignty and territorial integrity," China's Ministry of Foreign Affairs said in a statement.
“The Taiwan question is purely an internal affair of China and no other country is entitled to act as a judge on the Taiwan question.”
The US said it would not be intimidated by what it called Chinese "sabre-rattling" and Secretary of State Antony Blinken on Monday urged Beijing to "act responsibly" if Ms Pelosi proceeded with the visit.
In response to Ms Pelosi's visit, China's defence ministry said the military has been put on "high alert" and that it will conduct "targeted military operations".
The Eastern Theatre Command said the exercises will include joint air and sea drills to the north, south-west and north-east of the island, long-range live firing in the Taiwan Strait and missile test launches in the ocean to the east of the territory.
Taiwan said Ms Pelosi's trip demonstrates "rock solid" support from Washington and will strengthen the "friendly relations" between the territory and the US.
Barricades were placed outside the Grand Hyatt in Taipei where Ms Pelosi is expected to stay on Tuesday night. Demonstrators both supporting and protesting against her visit stood outside the hotel. Two buildings in the capital displayed the message "Welcome to Taiwan, Speaker Pelosi" in LED lights.
China had previously stated it would issue a military response should the House speaker visit Taiwan and President Xi Jinping warned US President Joe Biden in a call last week that he should not “play with fire” over the territory.
Mr Biden told the Chinese leader that “Congress is an independent branch of government and that Speaker Pelosi makes her own decisions”, but Beijing pushed back, suggesting Congress should abide by the US government's foreign policy.
“When the House speaker, being the third-highest ranking figure in the US government, flies on a US military plane to make a provocative visit to the Taiwan region, it is certainly not unofficial behaviour,” Hua Chunying, spokeswoman for China's Ministry of Foreign Affairs, said at a briefing in Beijing.
Russia, which has grown closer to China in recent years, called the trip a “provocation” aimed at pressuring the Chinese government.
“We consider a possible visit by Pelosi to Taiwan … as another provocative action by the United States aimed at exerting additional pressure on Beijing,” said Maria Zakharova, spokeswoman for Russia's Ministry of Foreign Affairs.
Citing intelligence, White House National Security spokesman John Kirby said before Ms Pelosi's trip that China was preparing for possible military provocations.
While the US did not fear a direct attack on the speaker's military aircraft, Washington said exercises conducted by China “increase the risk of miscalculation”.
Washington has stressed that a trip by the House speaker would not undermine the nation's “One China” policy. Though the US does send military equipment to Taiwan, it does not hold diplomatic relations with the island and recognises Beijing as the government of China.
On Thursday, she will meet South Korean National Assembly Speaker Kim Jin Pyo in Seoul for discussions on security, economic co-operation and the climate crisis. Ms Pelosi is also expected to visit Japan.
Agencies contributed to this report