Taiwan's military sent forces to watch a Chinese naval formation, led by the aircraft carrier Shandong, sailing through the Taiwan Strait on Monday, Taiwan's defence ministry said, as Beijing keeps up pressure ahead of elections next month.

Chinese aircraft carrier Shandong sails in Pacific Ocean waters, south of Okinawa prefecture, Japan.

The Shandong, commissioned in 2019, last transited the sensitive strait in early November. The latest sailing comes about one month before presidential and parliamentary elections on the island.

The ministry said in a statement that the formation sailed south through the strait but kept to the western side of the waterway's median line - the Chinese side of the unofficial barrier between the two.

China says the Taiwan Strait is not an international waterway and that it alone has sovereignty there, which both Taiwan and the United States dispute.

Taiwan dispatched "appropriate" forces to keep watch on the formation, the ministry added, without elaborating.

"The security and prosperity of the Indo-Pacific region are closely related to global development and stability, and are obligations and responsibilities that all parties in the region must share," the ministry said.

Taiwan's military will "prepare for war without seeking war", strengthen its self-defence capabilities, and respond to regional threats, it added.

China's defence ministry did not immediately respond to a request for comment.

During the past four years Taiwan has repeatedly complained of stepped-up Chinese military activity nearby, including fighters and drones crossing the strait's median line.

Last week, Taiwan reported a rare night mission by China's air force over the strait.

Taiwan is on high alert for Chinese activities, both military and political, ahead of its election, especially what Taipei views as Beijing's efforts to interfere in the ballot to get electors to vote for candidates China may prefer.

Vice President Lai Ching-te and running mate Hsiao Bi-khim from the ruling Democratic Progressive Party are leading in the polls. China views them as separatists and has rebuffed Lai's offers of talks.