China has said it was ending cooperation with the United States on key issues including climate change -- a move Washington decried as "fundamentally irresponsible" as relations between the two superpowers nosedive over Taiwan.

The damaging rift, which carries significant geostrategic risk, was triggered by Chinese fury over a visit by US House Speaker Nancy Pelosi to Taiwan, which Beijing claims as its territory and has vowed to retake -- by force if necessary.

It has since Thursday encircled the self-ruled, democratic island with a series of huge military drills that have been roundly condemned by the United States and other Western allies.

And Friday saw China's foreign ministry hit back further against the United States, suspending talks and cooperation on multiple agreements -- including on climate change.

The world's two largest polluters last year pledged to work together to accelerate climate action this decade, and vowed to meet regularly to tackle the crisis. But that deal now looks shaky in light of China's latest move.

"We believe that this is fundamentally irresponsible," National Security Council spokesman John Kirby told reporters in Washington.

"They think they're punishing us... they're actually punishing the whole world," Kirby said, adding that the only way to defuse tensions was for China to halt its "provocative" military exercises.

UN Secretary-General Antonio Guterres warned the two superpowers must continue to work together -- for the world's sake.

"For the secretary-general there is no way to solve the most pressing problems of all the world without an effective dialogue and cooperation between the two countries," his spokesman Stephane Dujarric said.

Taiwan Premier Su Tseng-chang said the "evil neighbour next door" had taken everyone by surprise with its willingness to "arbitrarily jeopardise the busiest waterways in the world" with its military exercises.

- 'Our motherland is powerful' -

Beijing has said its drills will continue until midday Sunday, and Taipei reported that 68 Chinese planes and 13 warships crossed the "median line" that runs down the Taiwan Strait on Friday.

A fighter jet flew over the Chinese island of Pingtan, prompting tourists to snap photos, according to AFP journalists on the scene. A Chinese military vessel sailing through the Taiwan Strait was also visible.

China's drills involved a "conventional missile firepower assault" in waters east of Taiwan, the Chinese military said.

And state broadcaster CCTV reported that Chinese missiles flew directly over Taiwan -- a major escalation if confirmed.

On Pingtan, local tourists proudly extolled Beijing's military might against its much smaller neighbour.

"Our motherland is powerful. We are not afraid of having war with Taiwan, the US or any country in the world," Liu, a 40-year-old tourist from Zhejiang province, told AFP.

"We hope to unify Taiwan soon," he added. "We don't want to start a war, but we are not afraid of others."

Wang, a businesswoman, was more sanguine about prospects for cross-strait ties.

"I hope China can unify Taiwan, but I don't want war," she said. "I hope this issue can be solved in a peaceful way."

- 'Significant escalation' -

The scale and intensity of China's drills have triggered outrage in the United States and other democracies, with the White House summoning China's ambassador to Washington Friday to rebuke him over Beijing's actions.

"The fact is, the speaker's visit was peaceful," US Secretary of State Antony Blinken said after talks with Southeast Asian foreign ministers at an ASEAN summit in Phnom Penh.

"There is no justification for this extreme, disproportionate and escalatory military response," he added.

China's foreign minister countered by warning Washington not to escalate tensions.

"America's habit is to create a problem and then use this problem to achieve its goals. But this approach will not work with China," Wang Yi told a press conference on the summit sidelines.

"We want to issue a warning to the US not to act rashly and not to create a bigger crisis."

Japan has lodged a formal diplomatic complaint against Beijing, with five of China's missiles believed to have landed in its exclusive economic zone.

On Friday, Japan's foreign ministry said China cancelled a planned bilateral meeting on the ASEAN summit sidelines.

And Australia -- which has a troubled relationship with China, its largest trading partner -- condemned the drills as "disproportionate and destabilising".

The manoeuvres are taking place along some of the world's busiest shipping routes, used to disseminate the global supply of vital semiconductors and electronic equipment produced in East Asia.

Taiwan sees attack coming

Taiwan accused the Chinese army of simulating an attack on its main island Saturday, as Beijing doubled down on its retaliation for US House Speaker Nancy Pelosi's visit to Taipei after announcing a suspension of cooperation with Washington on key issues.

Relations between the two superpowers have nosedived in the wake of Pelosi's trip to China's self-ruled neighbour -- which it claims as its territory -- prompting calls from the UN for an urgent de-escalation of tensions.

And Friday saw the environment become the latest victim of the geopolitical jousting, as Beiing said it would withdraw from a series of talks and cooperation agreements with Washington -- most notably on climate change and defence cooperation.

The world's two largest polluters had pledged to work together to accelerate climate action this decade and vowed to meet regularly to tackle the crisis -- a deal that now looks shaky.

Beijing on Saturday continued some of its largest-ever military drills around Taiwan -- exercises aimed at practicising a blockade and ultimate invasion of the island, analysts say.

Taipei said it observed "multiple" Chinese planes and ships operating in the Taiwan Strait, believing them to be simulating an attack on the self-ruled democracy's main island.

"Multiple batches of Communist planes and ships conducting activities around the Taiwan Strait, some of which crossed the median line," its defence ministry said in a statement, referring to a demarcation line that runs down the Taiwan Strait which Beijing does not recognise.

In a bid to show just how close China's forces have been getting to Taiwan's shores, Beijing's military overnight released a video of an air force pilot filming the island's coastline and mountains from his cockpit.

China's state broadcaster, CCTV, has reported that Chinese missiles have flown directly over Taiwan during the exercises -- a major escalation if confirmed.

But Taipei has remained defiant, insisting it would not be cowed by its "evil neighbour".

- 'Punishing the whole world' -

The scale and intensity of China's drills have triggered outrage in the United States and other democracies, with the White House summoning China's ambassador to Washington on Friday to rebuke him over Beijing's actions.

And Beijing's decision to withdraw from hard-won cooperation on climate change has now sparked wider fears about the future of the planet.

"It's obviously worrying and raises concerns," Alden Meyer, a senior associate at E3G, a climate-focused think tank, told AFP.

It's "impossible to address the climate emergency if the world's number one and number two economies and number one and number two emitters are not taking action," he said.

"And it's always preferable that they do that in a collaborative way."

National Security Council spokesman John Kirby told reporters in Washington the decision was "fundamentally irresponsible."

"They're actually punishing the whole world, because the climate crisis doesn't recognize geographic boundaries and borders," Kirby said.

"The world's largest emitter now is refusing to engage on critical steps necessary to combat the climate crisis."

UN Secretary-General Antonio Guterres warned the two superpowers must continue to work together -- for the world's sake.

"For the secretary-general, there is no way to solve the most pressing problems of all the world without an effective dialogue and cooperation between the two countries," his spokesman Stephane Dujarric said.

- 'The new normal' -

But with tensions over Taiwan having risen to their highest level in nearly 30 years with an elevated risk of military conflict, experts told AFP the latest downturn in relations between the two superpowers could be deep and long-lasting.

"The relationship is in a very bad place right now," said Bonnie Glaser, a China expert at the German Marshall Fund.

The suspension Friday of bilateral military and maritime dialogue while China continues its military exercises was "particularly worrisome,” she said.

"We don't know what else they will do," she said. "We just don't know if this is just a temporary thing."

John Culver, a former CIA Asia analyst, said in a discussion hosted by the Center for Strategic and International Studies that Beijing's main purpose with its military exercises was to change that status quo.

"I think that this is the new normal," Culver said.

"The Chinese want to show... that a line has been crossed by the speaker's visit."