"17 aircraft (of the 21) crossed the median line and its extension, entered our northern, central, and southwestern (air defence identification zone), and joined PLA vessels for joint combat patrol," it said in a statement.

Taiwan's armed forces "are monitoring the activities with our joint surveillance systems, and have dispatched appropriate assets to respond accordingly."

The median line bisects the Taiwan Strait, a narrow 180-kilometre (110-mile) waterway separating the island from mainland China.

Beijing does not recognise the line as it claims democratic Taiwan as part of its territory. It has also never renounced the use of force to bring the island under its control.

Under the administration of Taiwanese President Tsai Ing-wen, tensions between Beijing and Taipei have ramped up, as she and her government do not acknowledge China's claim.

Her deputy, Vice President Lai, won elections in January despite warnings from Beijing that he would be the cause of "war and decline" for Taiwan.

China regards Lai -- who used to be outspoken about Taiwan independence -- as a "dangerous separatist", though he has moderated his views in the past year.