China's military kicked off the war games Thursday morning, encircling Taiwan with naval vessels and military aircraft as it vowed the blood of "independence forces" on the island would flow.

The exercises were launched after Lai Ching-te took office as Taiwan's new president this week and made an inauguration speech that China denounced as a "confession of independence".

The drills are part of an escalating campaign of intimidation by China that has seen it carry out a series of large-scale military exercises around Taiwan in recent years.

They are testing the "capability of joint seizure of power, joint strikes and control of key territories", Li Xi, spokesman for the People's Liberation Army (PLA) Eastern Theater Command, said on Friday.

China, which split with Taiwan at the end of a civil war 75 years ago, regards the island as a renegade province with which it must eventually be reunified.

The dispute has long made the Taiwan Straits one of the world's most dangerous flashpoints, and this week's events have stoked fears that China may use military force to bring the island under mainland rule.

The United States, Taiwan's strongest ally and military backer, on Thursday "strongly" urged China to act with restraint. The United Nations called for all sides to avoid escalation.

- Cut 'blood vessels' -

As the drills -- Codenamed "Joint Sword-2024A" -- got under way, China said they would serve as "strong punishment for the separatist acts of 'Taiwan independence' forces".

Footage published by China's military showed soldiers streaming out of a building to battle stations and jets taking off to a rousing martial tune.

State broadcaster CCTV reported that Chinese sailors had called out to their Taiwanese counterparts at sea, warning them against "resisting reunification by force".

And an animated graphic published by the Chinese military showed missiles raining down on key targets in the north, south and east of the island, declaring it would "cut off the blood vessels for Taiwan independence!"

Taipei's coast guard on Friday reported four vessels from its Chinese counterpart entered the "restricted waters" of two Taiwanese islands as two others nearby provided support.

"This is the eighth time this month Chinese coast guard vessels have sailed into the restricted waters," Taipei's coast guard said, adding that the vessels left around 9 am (0100 GMT).

"We urge China to exercise self-restraint and immediately stop its irrational behaviour," it added.

China has repeatedly branded Lai a "dangerous separatist" who would bring "war and decline" to the island.

Lai said in a speech on Thursday he would "stand on the front line" to defend Taiwan, without directly referring to the ongoing drills.

Beijing was incensed by his inauguration speech on Monday, in which he hailed a "glorious" era for Taiwan's democracy.

- 'Heads broken' -

Chinese foreign ministry spokesman Wang Wenbin on Thursday delivered a warning that included language more commonly used by China's propaganda outlets.

"Taiwan independence forces will be left with their heads broken and blood flowing after colliding against the great... trend of China achieving complete unification," Wang told reporters.

Beijing's Xinhua news agency and ruling party organ the People's Daily ran editorials hailing the drills on Friday, while slamming Lai's "treacherous behaviour" and promising a "severe blow".

The drills are taking place in the Taiwan Strait and to the north, south and east of the island, as well as areas around the Taipei-administered islands of Kinmen, Matsu, Wuqiu and Dongyin.

Beijing has said the drills will last until Friday, but analysts say it could choose to extend the war games or launch missiles near Taiwan, as it did after a visit to the island by then-US House Speaker Nancy Pelosi in 2022.

Taipei's defence ministry said Thursday that the Chinese military had gotten within 24 nautical miles (44 kilometres) of Taiwan's main island.