The United States "strongly" urged China to show restraint after it launched military drills around Taiwan following the inauguration of a new president, describing China's actions as "reckless".
The US had "noted with concern" and was "closely monitoring" the drills that China said were punishment for Taiwanese President Lai Ching-te's inauguration speech, a senior US official said.
"We strongly urge Beijing to act with restraint," the Biden administration official said, warning China not to use Taiwan's political transition as a "pretext or excuse for provocative or coercive measures."
China's actions were "reckless, risk escalation, and erode longstanding norms that have maintained regional peace and stability for decades," the official added.
The United States, which recently approved billions in military aid for Taiwan, was "confident in our current force posture" in the region.
The Chinese drills came after Mr Lai said in his inaugural speech on Monday that Taiwan "must demonstrate our resolution to defend our nation".
China, which claims Taiwan as part of its territory, has denounced Mr Lai's speech as a "confession of independence."
The US and China have been trying to thaw their relations recently, with President Joe Biden and President Xi Jinping meeting in California last year. However, Taiwan remains a major source of tension.
A senior US general said earlier that the Chinese drills were expected but "concerning".
"We expected something like this, frankly," US Lieutenant General Stephen Sklenka, the US Indo-Pacific Command deputy commander, told an audience in Canberra.
"Just because we expect that behaviour does not mean that we should not condemn it - and we need to condemn it publicly," he said, adding that "it is concerning".
Lt Gen Sklenka said other nations besides the US should speak out against the drills.
Earlier, China encircled Taiwan with naval vessels and military aircraft in war games, as it vowed the blood of "independence forces" on the self-ruled island would flow.
The two days of drills are part of an escalating campaign of intimidation by China that has seen it carry out a series of large-scale military exercises around Taiwan in recent years.
As the drills got under way, China's military said they would serve as "strong punishment for the separatist acts of "Taiwan independence' forces".
Foreign ministry spokesman Wang Wenbin then delivered a warning that included language more commonly used by China's propaganda outlets.
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"Taiwan independence forces will be left with their heads broken and blood flowing after colliding against the great... trend of China achieving complete unification," Mr Wang told reporters.
China - governed by the Communist Party since 1949 - claims Taiwan as part of its territory and has vowed to bring the democratic island under its rule, by force if necessary.
The drills - codenamed "Joint Sword-2024A" - involve aircraft and ships surrounding the island to test their combat capabilities, China's People's Liberation Army (PLA) said.
Taiwan responded by deploying air, ground and sea forces, with the island's defence ministry vowing to "defend freedom".
President Lai said he would "stand on the front line" to defend Taiwan in a speech, without directly referring to the ongoing drills.
"Faced with external challenges and threats, we will continue to defend the values of freedom and democracy, and safeguard peace and stability in the region," he said.
China has repeatedly branded Lai a "dangerous separatist" who would bring "war and decline" to the island.
China was further incensed with his inauguration speech on Monday in which he hailed a "glorious" era for Taiwan's democracy.
The drills, which began this morning, are taking place in the Taiwan Strait and to the north, south and east of the island, as well as areas around the Taipei-administered islands of Kinmen, Matsu, Wuqiu and Dongyin.
China's military put out a series of posters touting what it called its "cross-strait lethality". They featured rockets, jets and naval vessels next to blood-stained text.
"The weapon aimed at 'Taiwan independence' to kill 'independence' is already in place," it declared.
As of around 8pm local time (1am Irish time), Taipei's defence ministry said 49 jets and planes had been detected since the drills began earlier that morning. Thirty-five of the aircraft had crossed the median line bisecting the Taiwan Strait.
Beijing, which split with Taipei at the end of a civil war 75 years ago, regards the self-ruled island as a renegade province with which it must eventually be reunified.
China has stepped up pressure on the island of 23 million people, periodically stoking worries about a potential invasion.
A Chinese military expert told CCTV that the drills were partly aimed at rehearsing an economic blockade of the island.
Zhang Chi, a professor at Beijing's China National Defense University, said the drills aimed to "strangle" Taiwan's critical Kaohsiung port to "severely impact" its foreign trade.
They would cut off "Taiwan's lifeline of energy imports" as well as "block the support lines that some US allies provide to 'Taiwan independence' forces", he added.
The last time China announced similar military exercises around Taiwan was in August last year after Lai, then vice president, stopped over in the United States on a visit to Paraguay.
Those drills also tested the PLA's ability "to seize control of air and sea spaces" and fight "in real combat conditions", according to state media.
They followed April drills that simulated the encirclement of the island, launched after Lai's predecessor Tsai Ing-wen met then-US House Speaker Kevin McCarthy in California.
China also launched major military exercises in 2022 after Nancy Pelosi, then the speaker of the US House of Representatives, visited Taiwan.
World powers are keen to see as much stability as possible between China and Taiwan, not least because of the vital role the island plays in the global economy.
The Taiwan Strait is one of the world's most important maritime trade arteries, and the island itself is a major tech manufacturer, particularly of vital semiconductors - the tiny chips used in everything from smartphones to missile systems.
Read more:
China's shows of force against Taiwan
The United States switched its diplomatic recognition from Taiwan to China in 1979 but remains the island's most important ally and supplier of military hardware.
US President Joe Biden has said he does not support Taiwan's independence but also that he would back sending forces to defend the island. The official US position on intervention is one of ambiguity.
The United States did not give an immediate official response to the drills.
US Lieutenant General Stephen Sklenka, speaking in Canberra, described the exercises as "concerning" but not unexpected.
