China has branded a landmark visit to Taiwan by US Speaker Nancy Pelosi as "extremely dangerous".
It accused Ms Pelosi, the most senior US politician in 25 years to visit the island China claims as its own, of "playing with fire".
"Those who play with fire will perish by it," Beijing warned in a statement.
Ms Pelosi said her visit "honours America's unwavering commitment to supporting Taiwan's vibrant democracy" and did not contradict US policy.
As her plane touched down, Chinese state media reported that its military jets were crossing the Taiwan strait. Taiwan denied those reports at the time - but later said that more than 20 Chinese military planes had entered its air defence zone on Tuesday.
China - which sees Taiwan as a breakaway province which will one day reunite with it - has previously warned that its armed forces "will not stand idly by".
Within an hour of the plane's arrival, it announced that the People's Liberation Army will conduct a series of live-fire military drills in the air and at sea around Taiwan later this week - warning ships and aircraft not to enter the affected areas.
It follows days of escalating tensions ahead of the visit, in which Chinese warplanes had already ventured out as far as the median line, the unofficial dividing line separating China and Taiwan in the waters between them.
In her statement, Ms Pelosi said: "America's solidarity with the 23 million people of Taiwan is more important today than ever, as the world faces a choice between autocracy and democracy."
And in an article published in the Washington Post newspaper at the same time, Ms Pelosi also wrote that Taiwan's "robust democracy... is under threat".
"In the face of the Chinese Communist Party's (CCP) accelerating aggression, our congressional delegation's visit should be seen as an unequivocal statement that America stands with Taiwan, our democratic partner, as it defends itself and its freedom," she said.
After her arrival, White House national security spokesperson John Kirby told reporters that the trip was consistent with the US's longstanding policy towards China and did not violate the country's sovereignty.
"There's no reason for this visit to become a spurring event for a crisis or conflict," he said.
While Ms Pelosi's visit had been the subject of vast international speculation for days, it had been shrouded in secrecy up until the last minute.
When she set off on a tour of Asia on Sunday, there was no mention of Taiwan on her official itinerary, which included Singapore, Malaysia, South Korea and Japan.
The White House has been open in its opposition to any such trip, and President Joe Biden said the military assessed it as "not a good idea".
But after Ms Pelosi landed, White House spokesman John Kirby told CNN this visit was similar to previous trips by other officials.
"There is no reason for this to erupt into conflict. There's no change to our policy. This is absolutely consistent with it."
Addressing the strong reaction from China, he said: "The United States is not going to be intimidated by threats."