Taiwan's defense ministry said it had detected on Wednesday evening 18 Chinese air force planes operating around Taiwan and carrying out "joint combat readiness patrols" with Chinese warships, the first large-scale post-election military activity.
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 defense ministry said that starting around 7.50 pm (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.
Median line used to be a barrier, but now China crosses it regularly
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 recognize the line's existence.
Taiwan sent its own forces to monitor, the 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-defense capabilities in accordance with enemy threats and self-defense needs, and respond to regional threats."
There was no immediate response from China's defense 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.
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.