BEIJING - The ongoing military drills around Taiwan are aimed at rehearsing a possible future invasion of the self-ruled island, Chinese military observers and state media have said.

The drills, which have for the first time involved firing ballistic missiles directly over the island, are also a thinly veiled threat against those pushing for formal independence of Taiwan.

Beijing declared four days of military drills, beginning on Thursday (Aug 4), after US House Speaker Nancy Pelosi visited Taiwan this week.

The drills are taking place in six zones that encircle the island. Chinese experts say these were chosen to simulate a blockade of Taiwan and a possible future campaign against it.

"(The drills) have created this encirclement of Taiwan. This will help us create beneficial strategic conditions for reunification. It will also stop external forces from interfering," said Professor Meng Xiangqing of the National Defence University, an institution administered by China's People's Liberation Army (PLA).

He added that this was the first time missiles have been fired directly over Taiwan.

China views Taiwan as a renegade province to be retaken, by force if necessary.

The United States, which maintains a policy of strategic ambiguity towards Taiwan, has not made clear if it would defend the island in the event of an invasion. But under its Taiwan Relations Act, Washington is obliged to provide the island with the means to defend itself.

The two zones off the northern coast of Taiwan will "seal off" the major port of Keelung, which is also close to the Japanese island of Okinawa, where US forces are based, said Prof Meng, who was speaking to Chinese state broadcaster CCTV.

Another exercise zone to the east of Taiwan could be used as staging grounds to launch strikes against Taiwan's military bases in Hualien and Taitung, while the zone to the west of Taiwan is located at the narrowest part of the Taiwan Strait, said Prof Meng.

The zone south-east of Taiwan could be used to blockade the Bashi Channel, a key waterway through which ships must pass to enter the South China Sea.

The final zone, located near Kaohsiung - a major city and also where a major base is situated - also encroaches into Taiwan's territorial waters, an area within 12 nautical miles of its coast.

It would effectively "create conditions to shut the door and beat the dog", Prof Meng said, using a Chinese saying that means blocking an enemy's escape route.

The drills have also featured some of Beijing's latest weaponry, including the J-20 stealth fighters and DF-17 hypersonic missiles, reported Chinese state media.

Both the scale and proximity of the exercises are unprecedented. State news agency Xinhua said more than 100 aircraft, including fighter jets and bombers, and over 10 warships have been activated.

Experts also point out that all six of the drill areas are across the median line of the Taiwan Strait, the unofficial line between mainland China and Taiwan.

The mainland does not formally recognise the boundary but has generally not crossed it. But this has been changing over the past two years, as incursions across the median line by PLA air force jets have become more common.

Analysts worry that such military movements will become normalised, raising the risk of conflict.

Shanghai-based military analyst Ni Lexiong said exercises of such proximity to Taiwan will become more common in future, as the mainland dials up the pressure on the island.

"If there is to be war, you must make preparations, and exercises are a key part of that," said Dr Ni.