When a group of Chinese warplanes simulated an attack on the US aircraft carrier last week, Beijing was delivering a warning.

The USS Theodore Roosevelt was entering the South China Sea, disputed waters that Beijing claims as its own but where Washington insists on asserting freedom of navigation for everyone else, when six Chinese H-6K heavy bombers flanked by four J-16 fighter jets flew into Taiwan’s air defence zone.

“That manoeuvre killed three birds with one stone: they signalled to the Americans that they regard the US military as a destabilising force here; they intimidated Taiwan; and they got some valuable practice at the same time,” said a Taiwanese former senior military commander.

Over the past year, these operations have become a regular feature for China’s People’s Liberation Army in the Taiwan Strait.

In October, Yen Teh-fa, Taiwan’s defence minister, told lawmakers that the PLA had flown into Taiwan’s air defence zone 253 times since the beginning of 2020. Taiwan reported another 230 such incursions, a steep increase, in the following months.

The escalation in Chinese air force activity has been widely interpreted as a sign of the unprecedented level of tensions between Beijing, which claims Taiwan as part of its territory and retains a threat to take it by force, and Taipei.

But defence experts believe two bigger forces are at play: the open military rivalry between Washington and Beijing; and the Chinese military’s need for a location to train for ambitious and far-flung missions and practise with increasingly sophisticated equipment.

Taiwan, however, is at greater risk because China, its main enemy, and the US, its unofficial security guarantor, have drifted towards confrontation — a course that analysts believe the Biden administration will struggle to alter.

The majority of Chinese military flights enter the south-west corner of Taiwan’s Air Defence Identification Zone. This area between Pratas, a Taiwan-held atoll in the South China Sea, and Taiwan’s south-western coast sits at the crossroads of the Taiwan Strait and the Bashi Channel, the main waterway connecting the South China Sea to the western Pacific.

“They are coming partly because the Americans are coming,” said a senior Taiwanese government official. “The US military has been sending reconnaissance aircraft through the Bashi Channel with increasing frequency and the Chinese are countering that.”

According to the South China Sea Strategic Situation Probing Initiative, a project backed by Chinese universities, large US reconnaissance aircraft flew over the South China Sea 70 times in January.

Heino Klinck, a China military expert who was the Pentagon’s deputy assistant secretary of defence for east Asia until last month, said the Chinese air force and navy aerial operations around Taiwan had expanded in scope and scale.

“The PLA has become increasingly threatening and aggressive with the effect of destabilising the entire region,” Mr Klinck told the Financial Times.

“In recent months, we have seen increases in volume, frequency, and composition of PLA intrusions into Taiwan air space particularly during and after high-level visits by US officials.

The US navy has challenged China’s campaign to build up and militarise land features in the South China Sea since 2015, with so-called Freedom of Navigation Operations. But over the past year, it has stepped up its activities with larger operations, including exercises involving aircraft carriers.

Apart from shadowing and pushing back against these operations, defence experts said China had seized them as an opportunity for exercises it needed to conduct.

“In that south-west corner of our ADIZ, the shallow waters of the Taiwan Strait drop off into the deep water of the South China Sea. It is an environment exceptionally rich in features they need to study for submarine and anti-submarine warfare,” said Su Tzu-yun, an analyst at the Institute for National Defence and Security Studies, a think-tank backed by Taiwan’s defence ministry.

Analysts note that while Chinese nuclear-powered submarines are capable of reaching the US west coast, to do so they must sail from their base on the island of Hainan through the Bashi Channel into the west Pacific, where they will be tracked by US submarines.

Chinese aircraft have been flying over the waters the submarines would have to sail on an almost daily basis in recent months. The area will also serve as a staging post for the submarines Taiwan is building in the southern port of Kaohsiung when they enter service from 2023.

A third of the 230 Chinese sorties flown into Taiwan’s air defence zone since September were anti-submarine warfare aircraft.

The heightened air activity imposes a big cost on Taiwan. The country’s air force, which will have to wait for another three years until the first of 66 F-16 fighters bought from the US start arriving, is forced to scramble its ageing jets constantly to discourage Chinese aircraft from drawing closer.

Said the senior Taiwanese official: “This is certainly another advantage the Chinese military sees in doing these operations.”