Taiwan said on Saturday that China's military drills appear to simulate an attack on the self-ruled island after multiple Chinese warships and aircraft crossed the median line of the Taiwan Strait following US House Speaker Nancy Pelosi's visit to Taipei that infuriated Beijing.

Taiwan's armed forces issued an alert, dispatched air and naval patrols around the island, and activated land-based missile systems in response to the Chinese exercises, the Ministry of National Defense said. As of 5pm Taiwan time 20 Chinese aircraft and 14 ships continued to carry out the sea and air exercises around the Taiwan Strait, it said.

The ministry said that zones declared by China as no-go areas during the exercises for other ships and aircraft had "seriously damaged the peace." It emphasised that Taiwan's military does not seek war, but would prepare and respond to it accordingly.

China's Ministry of Defence said in a statement on Saturday that it had carried out military exercises as planned in the sea and airspaces to the north, southwest and east of Taiwan, with a focus on "testing the capabilities" of its land strike and sea assault systems.

China launched live-fire military drills following Pelosi's trip to Taiwan earlier this week, saying it violated the "one-China" policy. China sees the island as a breakaway province to be annexed by force if necessary and considers visits to Taiwan by foreign officials as recognising its sovereignty.

Taiwan's army also said it detected four unmanned aerial vehicles flying in the vicinity of the offshore county of Kinmen and fired warning flares in response.

The four drones, which Taiwan believed were Chinese, were spotted over waters around the Kinmen island group and the nearby Lieyu Island and Beiding islet, according to Taiwan's Kinmen Defense Command.

Kinmen, also known as Quemoy, is a group of islands only 10 km east of the Chinese coastal city of Xiamen in Fujian province in the Taiwan Strait, which divides the two sides that split amid civil war in 1949.

"Our government & military are closely monitoring China's military exercises & information warfare operations, ready to respond as necessary," Taiwan's President Tsai Ing-wen said in a tweet.

"I call on the international community to support democratic Taiwan & halt any escalation of the regional security situation," she added.

A People's Liberation Army member looks through binoculars during military exercises as Taiwan's frigate Lan Yang is seen in the distance. Photo / AP

The Chinese military exercises began and are expected to last until Sunday. So far, the drills have included missile strikes on targets in the seas north and south of the island in an echo of the last major Chinese military drills in 1995 and 1996 aimed at intimidating Taiwan's leaders and voters.

Taiwan has put its military on alert and staged civil defence drills, while the US has deployed numerous naval assets in the area.

The Biden administration and Pelosi have said the US remains committed to a "one-China" policy, which recognises Beijing as the government of China but allows informal relations and defence ties with Taipei. The administration discouraged but did not prevent Pelosi from visiting.

China has also cut off defence and climate talks with the US and imposed sanctions on Pelosi in retaliation for the visit.

Pelosi said in Tokyo, the last stop of her Asia tour, that China will not be able to isolate Taiwan by preventing US officials from travelling there.

Pelosi has been a long-time advocate of human rights in China. She, along with other lawmakers, visited Beijing's Tiananmen Square in 1991 to support democracy two years after a bloody military crackdown on protesters at the square.

Meanwhile, cyberattacks aimed at bringing down the website of Taiwan's Ministry of Foreign Affairs had doubled, compared to similar attacks ahead of Pelosi's visit, according to Taiwan's Central News Agency. The ministry did not specify the origin of the attack.

Other ministries and government agencies, such as the Ministry of Interior, also faced similar attacks on their websites, according to the report.

A distributed-denial-of-service attack is aimed at overloading a website with requests for information that eventually crashes it, making it inaccessible to other users.

Also, the Central News Agency reported that the deputy head of the Taiwan Defence Ministry's research and development unit, Ou Yang Li-hsing, was found dead in his hotel room after suffering a heart attack. He was 57 and had supervised several missile production projects.

The report said his hotel room in the southern county of Pingtung, where he was on a business trip, showed no signs of intrusion.

Taiwanese overwhelmingly favour maintaining the status quo of the island's de facto independence and reject China's demands that the island unifies with the mainland under Communist control.

Globally, most countries subscribe to the "one-China" policy, which is a requirement to maintain diplomatic relations with Beijing.

Any company that fails to recognise Taiwan as part of China often faces swift backlash, often with Chinese consumers pledging to boycott its products.

Mars Wrigley, the manufacturer of the Snickers candy bar, apologised after it released a video and materials featuring South Korean boy band BTS that had referred to Taiwan as a country, drawing swift criticism from Chinese users.

In a statement on its Weibo account, the company expressed "deep apologies.

"Mars Wrigley respects China's national sovereignty and territorial integrity and conducts business operations in strict compliance with local Chinese laws and regulations," the statement said.

In a separate post, the firm added that there is "only one China" and said that "Taiwan is an inalienable part of China's territory."