Taiwan scrambled fighter jets on Friday as 18 Chinese aircraft buzzed the island, crossing the sensitive mid-line of the Taiwan Strait, in response to a senior US official holding talks in Taipei.

China had earlier announced combat drills and denounced what it called collusion between the island, which it claims as part of its territory, and the US.

US undersecretary for economic affairs Keith Krach arrived in Taipei on Thursday for a three-day visit, the most senior state department official to come to Taiwan in four decades — to which China had promised a “necessary response”.

Beijing has watched with growing alarm the ever-closer relationship between Taipei and Washington, and has stepped up military exercises near the island, including two days of large-scale air and sea drills last week.

Taiwan said 18 Chinese aircraft were involved on Friday, far more than in previous such encounters.

“Sep. 18, two H-6 bombers, eight J-16 fighters, four J-10 fighters and four J-11 fighters crossed the midline of the TaiwanStrait and entered Taiwan’s southwest ADIZ,” the defence ministry said in an English-language tweet.

“ROCAF scrambled fighters, and deployed air defence missile system to monitor the activities.” The ROCAF is Taiwan’s air force.

In Beijing, Chinese defence ministry spokesman Ren Guoqiang said Friday’s manoeuvres, about which he gave no details, involved the People’s Liberation Army’s eastern theatre command. “They are a reasonable, necessary action aimed at the current situation in the Taiwan Strait and protecting national sovereignty and territorial integrity,” Ren said.