Taiwan’s air force scrambled into action on Sunday after spotting 10 Chinese warplanes crossing the sensitive median line of the Taiwan Strait, as the island’s defence ministry said four Chinese warships also carried out combat patrols.

This is the second time in less than a week that Taiwan has reported renewed Chinese military activity, after 37 Chinese military aircraft on Thursday flew into the island’s air defence zone, some of which then flew into the western Pacific.

China, which views democratically governed Taiwan as its own territory, has over the past three years regularly flown its air force into the skies near the island, though not into Taiwan’s territorial air space.

In a short statement, Taiwan’s defence ministry said that as of 0600 GMT on Sunday it had detected 24 Chinese air force planes, including J-10, J-11, J-16 and Su-30 fighters, as well as H-6 bombers.

It did not specify where the aircraft flew but said 10 had crossed the median line of the Taiwan Strait, which separates the two sides and had previously served as an unofficial barrier.

China says it does not recognise that and has been routinely crossing it since last year.

Four Chinese naval ships were also engaged in “joint combat readiness patrols”, the ministry added, without giving details.

Taiwan sent up its own fighters and deployed ships and land-based missile systems to keep watch, it said, using typical wording for how it responds to such Chinese activities.

China’s defence ministry did not immediately respond to requests for comment. It has not commented on Thursday’s flights.

China has previously said such missions are to protect the country’s sovereignty and aimed at “collusion” between Taiwan and the United States, the island’s most important international backer and arms seller.

In April, China held war games around Taiwan following a trip to the United States by Taiwan President Tsai Ing-wen.

Taiwan’s government rejects China’s sovereignty claims and says only the island’s people can decide their future.

China has never renounced the use of force to bring Taiwan, which is gearing up for presidential and parliamentary elections in January, under its control.

In a Sunday video address to supporters on the Taiwan-controlled Matsu islands close to the Chinese coast, Taiwan’s Vice President William Lai said he would do his best to “stabilise the peaceful status quo in the Taiwan Strait” if he wins the presidency, his campaign office said.

Lai is running as the ruling Democratic Progressive Party’s candidate. Tsai cannot run again due to term limits.

Tsai has repeatedly offered talks with China but been rebuffed as Beijing views her and her party as separatists.

Laura Rosenberger, chair of the American Institute in Taiwan, which manages the unofficial relationship between Washington and Taipei, visited Taiwan last week and meet all three presidential candidates.