Taiwan's defence ministry said it detected 18 Chinese air force planes operating around Taiwan and carrying out "joint combat readiness patrols" with Chinese warships on Wednesday, the first large-scale military activity after the Taiwanese election.

China, which views Taiwan as its own territory, has over the past four years regularly sent warplanes and warships into the skies and waters around the island as it seeks to assert sovereignty claims that the Taipei government rejects.

Taiwan voted for the ruling Democratic Progressive Party's (DPP) Lai Ching-te as its next president on Saturday, a man Beijing has repeatedly blasted as a dangerous separatist and bringer of war.

Taiwan's defence ministry said that starting around 7:50 p.m. (1150 GMT) on Wednesday it had detected 18 aircraft including Su-30 fighters operating off northern and central Taiwan and to the island's southwest.

Eleven of those aircraft crossed the Taiwan Strait's median line, or areas close by, working with Chinese warships to carry out "joint combat readiness patrols", the ministry added.

The strait's median line once served as an unofficial barrier between the two sides, but Chinese planes now regularly fly over it. China says it does not recognise the line's existence.

Taiwan sent its own forces to monitor, its defence ministry said.

"The security and prosperity of the Taiwan Strait region are closely related to global development and stability, and are obligations and responsibilities that all parties in the region must share," it said in a statement.

"The military will continue to strengthen its self-defence capabilities in accordance with enemy threats and self-defence needs, and respond to regional threats."

There was no immediate response from China's defence ministry.

Earlier on Wednesday, China's Taiwan Affairs Office said Beijing's position that it would not renounce using force to bring Taiwan under its control was aimed at foreign interference and a tiny number of separatists, but added that Taiwanese needed to be disabused of "biases" against China.

A U.S. State Department spokesperson said the United States was closely monitoring Beijing's actions and urged it not to use Taiwan's election as "pretext for escalation."

"We have consistently urged restraint and no unilateral change to the status quo, which has preserved peace and stability in the Taiwan Strait and throughout the region for decades," the spokesperson said.

One source familiar with the Biden administration's thinking said "it would not be surprising to see Beijing use the next few months to gradually ratchet up pressure on Taiwan."

"We've already seen diplomatic pressure and threats of further economic pressure. And that pressure could continue even after the inauguration," the person said on condition of anonymity.

Lai, who takes office on May 20, has repeatedly offered talks with China but has been rebuffed. He says he will maintain peace and stability across the strait, but that only Taiwan's people can decide its future.

Vincent Chao, spokesperson for Lai's campaign and head of international affairs for the DPP, told the U.S. Institute of Peace think tank that the new administration would be one of "continuation" and "no surprises", while maintaining strong deterrence along with the U.S. and other countries in the region.

He said the administration would make every effort to be responsible and pragmatic and avoid provocations, while maintaining that deterrence.

"We must never give an opportunity for (Chinese leader) Xi Jinping to wake up one day and decide that today's a good day," he said.

"It's all based on risk and cost ... And so the risk and cost of him taking action must be astronomically high," Chao said, while adding: "We're also committed to keeping the risk and costs of not taking action very, very low."