The US said that China's launch of nine ballistic missiles around Taiwan was an "irresponsible" overreaction to the visit of US House Speaker Nancy Pelosi to the island.
"China has chosen to overreact and use the speaker's visit as a pretext to increase provocative military activity in and around the Taiwan Strait," said White House spokesman John Kirby.
He added that US aircraft and naval vessels would remain on station in the general area to monitor the situation.
"We will not be deterred from operating in the seas and the skies of the Western Pacific consistent with international law, as we have for decades, supporting Taiwan and defending a free and open Indo-Pacific," Mr Kirby said.
China deployed scores of planes and fired live missiles near Taiwan in its biggest-ever drills in the Taiwan Strait, a day after US House of Representatives Speaker Nancy Pelosi made a solidarity trip to the self-ruled island.
China's military confirmed multiple firings of conventional missiles in waters off Taiwan as part of planned exercises in six zones set to run until noon on Sunday. It activated more than 100 planes, including fighter jets and bombers, and over 10 warships, state broadcaster CCTV said.
Taiwan's defence ministry said it scrambled jets to warn away 22 Chinese fighter aircraft that crossed the Taiwan Strait median line into its air defence zone, while Japan protested that five missiles appeared to land in its economic zone.
"The US-Taiwan collusion and provocation will only push Taiwan towards the abyss of disaster, bringing catastrophe to Taiwan compatriots," said a Chinese defence ministry spokesperson.
Responding to the Chinese drills, President Tsai Ing-wen said Taiwan would not provoke conflicts but would firmly defend its sovereignty and national security.
"Taiwan will never be knocked down by challenges," Tsai said in a recorded video message to the people of Taiwan.
"We are calm and not impetuous, we are rational and not provocative, but we will also be firm and not shirk."
Taiwan said 11 Chinese Dongfeng ballistic missiles had been fired in nearby waters - the first time since 1996.
Taiwan officials said the drills violated United Nations rules, invaded its space and threatened free air and sea navigation.
It has been self-ruled since 1949, when Mao Zedong's communists took power in Beijing after defeating Chiang Kai-shek's Kuomintang (KMT) nationalists in a civil war, prompting the KMT-led government to retreat to the island.
The military activity followed Ms Pelosi's unannounced visit of support to Taiwan in defiance of warnings from China.
Before the drills officially began, Chinese navy ships and military aircraft briefly crossed the Taiwan Strait median line several times this morning, a Taiwanese source briefed on the matter told Reuters.
By midday, warships from both sides remained in close proximity as Taiwan also scrambled jets and deployed missile systems to track Chinese aircraft crossing the line.
"They flew in and then flew out, again and again. They continue to harass us," the Taiwanese source said.
China, which has long said it reserves the right to take Taiwan by force, says its differences with the island are an internal affair.
In Taiwan, life was largely normal despite worries that Beijing could even fire a missile over the main island as North Korea did over Japan's northern island of Hokkaido in 2017.
"When China says it wants to annex Taiwan by force, they have actually said that for quite a while," said Chen Ming-cheng, a 38-year-old realtor. "From my personal understanding, they are trying to deflect public anger, the anger of their own people, and turn it onto Taiwan."
Taiwan said that websites of its defence ministry, foreign ministry and the presidential office were attacked by hackers, and warned of coming "psychological warfare".
China calls Pelosi visit 'highly irrational'
Chinese Foreign Minister Wang Yi called Ms Pelosi's visit to Taiwan a "manic, irresponsible and highly irrational" act, state broadcaster CCTV reported.
Wang, speaking at a meeting of Southeast Asian foreign ministers in Cambodia, said China had tried to avert crisis by diplomatic means but would never let its core interests be hurt.
Unusually, the drills in six areas around Taiwan were announced with a locator map circulated by China's official Xinhua news agency - a factor that for some analysts illustrated playing to both domestic and foreign audiences.
In Beijing, security near the US Embassy was unusually tight though there were no signs of significant protests.
"I think this (Pelosi's visit) is a good thing," said a man surnamed Zhao in Beijing. "It gives us an opportunity to surround Taiwan, then to use this opportunity to take Taiwan by force. I think we should thank Comrade Pelosi."
Ms Pelosi, the highest-level US visitor to Taiwan in 25 years, praised its democracy and pledged American solidarity during her brief stopover. Chinese anger could not stop world leaders from travelling there, she said.
"Our delegation came to Taiwan to make unequivocally clear that we will not abandon Taiwan," Ms Pelosi told Taiwan's President Tsai Ing-wen, whom Beijing suspects of pushing for formal independence - a red line for China.
China summoned the US ambassador in Beijing in protest and halted several agricultural imports from Taiwan.
The United States and the foreign ministers of the Group of Seven nations warned China against using Ms Pelosi's visit as a pretext for military action against Taiwan.
The United States has no official diplomatic relations with Taiwan but is bound by US law to provide it with the means to defend itself. Taiwan rejects China's sovereignty claims, saying only the islanders themselves can decide their future.
Pelosi lands in Japan in final stop on Asia trip
Ms Pelosi has arrived in Japan for the final stop on her Asian tour, having left South Korea earlier today.
It is her first trip to Japan since 2015, and she will meet Prime Minister Fumio Kishida for breakfast tomorrow, Japan's foreign ministry said, to discuss the two countries' alliance and issues of shared interest.
Ms Pelosi is also scheduled to discuss international affairs with Japan's House of Representatives speaker Hiroyuki Hosoda.
Japan, a key US ally, has lodged a diplomatic protest with China over its massive military exercises encircling Taiwan.
Just before Ms Pelosi's arrival, Defence Minister Nobuo Kishi said five ballistic missiles fired by China were believed to have landed in Japan's exclusive economic zone.
Parts of Japan's southernmost island region Okinawa are close to Taiwan, as are islets at the centre of a long-running dispute between Tokyo and Beijing.
During her visit to South Korea, Ms Pelosi discussed the "grave situation" and growing threat posed by North Korea's nuclear weapons programmes with her South Korean counterpart, National Assembly Speaker Kim Jin-pyo.
At a brief press conference in Seoul, during which she did not take any questions, Ms Pelosi hailed the "special" relationship between South Korea and the United States - but made no mention of her visit to Taipei.
"The US-Republic of Korea relationship is special to us," she said, adding that the bond which was forged during the Korean war "from urgency and security... has become the warmest of friendships."