Taiwan's coast guard released images it said showed Chinese vessels sailing north in rough seas past the island on Thursday, on their way to China.

"All the Chinese coast guard went back to China yesterday, thus, although they haven't officially made any announcement, we consider it over," Hsieh Ching-chin, deputy director general of Taiwan's coast guard, told AFP.

Beijing has not confirmed the drills and its defence ministry did not say whether the manoeuvres had taken place when asked at a press conference on Friday.

But ministry spokesman Wu Qian said that "whether or not we hold exercises, and when we hold them, are decided by us alone, based on our own needs and the circumstances of our struggle", according to an official social media account of the armed forces.

"Safeguarding national sovereignty and territorial integrity, the fundamental interests of the Chinese nation, and the common interests of compatriots across the Taiwan Strait are the (military's) sacred duties," Wu said.

"No matter whether it holds exercises, the People's Liberation Army will not be absent or soft-hearted when it comes to striking down (Taiwanese) 'independence' and pushing for unification," he said, referring to the Chinese armed forces.

Taiwanese authorities said this week that Beijing's biggest maritime drills in years stretched from near the southern islands of Japan to the South China Sea.

About 90 Chinese warships and coast guard vessels took part in the exercises, which included simulating attacks on foreign ships and practising blockading sea routes, a Taiwan security official said Wednesday.

There was no announcement by Beijing's army or Chinese state media about increased military activity in the East China Sea, Taiwan Strait, South China Sea or Western Pacific Ocean.

However, Taiwanese President Lai Ching-te's recent visit to the United States and a call with US House Speaker Mike Johnson sparked fury in Beijing.

While Taiwan regards itself as a sovereign nation, China claims the self-ruled island as part of its territory and opposes any international recognition of it.

- 'Red line' -

The Taiwan security official said China began planning the massive maritime operation in October and aimed to demonstrate it could choke off Taiwan while also drawing a "red line" ahead of the next US administration.

As Chinese ships returned to China on Thursday, Taiwan's military and coast guard closed emergency response centres set up in response to the huge maritime mobilisation.

"The nine ships went back to Chinese ports last night in two groups," Hsieh said of vessels that had been in waters to the southeast and southwest of Taiwan.

A defence ministry spokeswoman said Friday that Chinese warships and coast guard vessels had been detected returning to China.

The de facto US embassy in Taiwan said Thursday it was monitoring "with concern" Beijing's military activity near the island and in the region.

A US military patrol plane flew south to north over the sensitive waters separating Taiwan and China on Friday morning, Taiwan's defence ministry said.

Beijing has ramped up the deployment of fighter jets and warships around Taiwan in recent years as it pressures Taipei to accept its claims of sovereignty.

China has refused to rule out using force to bring Taiwan under its control, leaving the island to face the constant threat of invasion.

The latest exercise exceeded Beijing's maritime response to then-US House speaker Nancy Pelosi's visit to Taipei in 2022, the security official said. Those war games were China's largest ever around Taiwan.

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