China has been showing a new intensity and military sophistication as it steps up its harassment of Taiwan, with record numbers of military flights near the island over the last week.

China's People's Liberation Army flew 56 planes in international airspace off the southwest coast of Taiwan on Monday, setting a new record and capping four days of sustained pressure involving 149 flights.

The actions came as China, with growing diplomatic and military power, faces greater pushback from countries in the region and as Taiwan pleads for more global support and recognition.

The US called China's latest actions "risky" and "destabilising", while China - which claims Taiwan as its own and asserts its territorial ambitions in the region - responded that America selling weapons to Taiwan and its ships navigating the Taiwan Strait were provocative.

At the same time as the flights, the US stepped up naval manoeuvres in the Indo-Pacific with its allies, challenging Beijing's territorial claims in critical waterways.

Taiwanese defence minister Chiu Kuo-cheng said that the situation "is the most severe in the 40 years since I've enlisted".

While most agree that war is not imminent, Taiwanese president Tsai Ing-wen warned that more was at stake if Beijing made good on past threats to seize the island by force if necessary.

"If Taiwan were to fall, the consequences would be catastrophic for regional peace and the democratic alliance system," she wrote in an impassioned article in Foreign Affairs magazine.

"It would signal that in today's global contest of values, authoritarianism has the upper hand over democracy."

China regularly flies military aircraft into Taiwan's "air defence identification zone" - international airspace that Taiwan counts as a buffer in its defence strategy - although previous flights have usually involved a handful of planes at most.

Perhaps more significant than the number of planes was the make-up of the group, with fighters, bombers and airborne early warning aircraft, Euan Graham, a defence analyst with the International Institute for Strategic Studies in Singapore, said.

"That's the level of sophistication - it looks like a strike package, and that's part of the step-up in pressure," he said.

"This is not a couple of fighters coming close and then going straight back after putting one wing across the median; this is a much more purposeful manoeuvre."

Controlling Taiwan and its airspace is key to China's military strategy, with the area where the most recent sorties took place also leading to the west Pacific and the South China Sea.

The latest manoeuvres bring the total number of flights to more than 815 as of Monday since the Taiwanese government started publicly releasing the numbers a little more than a year ago.

China has been rapidly improving and strengthening its military, and the most recent flights demonstrate a greater level of technical expertise and power, Chen-Yi Tu, a researcher at the Institute for National Defence and Security Research in Taiwan, said.

Several democracies have been increasingly vocal in their support of Taiwan and have stepped up naval operations in the area.

As China was conducting its most recent flights, 17 ships from six navies - the US, Britain, Japan, the Netherlands, Canada and New Zealand, including three aircraft carriers and a Japanese helicopter carrier - carried out joint manoeuvres off the Japanese island of Okinawa, northeast of Taiwan, meant to show their commitment to a "free and open Indo-Pacific".

A few days earlier, the British frigate HMS Richmond transited through the Taiwan Strait, announcing its presence on Twitter and angering China, which condemned the move as a "meaningless display of presence with an insidious intention".

The international actions are an attempt to counter China's frequent claim that its own actions are in response to American moves, and demonstrate that democracies intend to defend established maritime laws and norms, Mr Graham said.

"When the UK sends a ship through the Taiwan Strait for the first time since 2008 and it sailed down the median line, the point that it's making is that they know China knows where that line is," he said.

"In order for the status quo to be meaningful, it has to be upheld and the most emphatic way to do that is to physically demonstrate with a government asset like a warship."

J Michael Cole, a Taipei-based senior fellow with Global Taiwan Institute in Washington, said: "We are seeing a slow emergence of some sort of coalition of democracies in the region that are trying to come together to build some sort of mechanism to respond to Chinese behaviour in the region."

Under longstanding policy, the United States provides political and military support for Taiwan, but does not explicitly promise to defend it from a Chinese attack.