TAIPEI/BEIJING  - Taiwan scrambled fighter jets on Friday as multiple Chinese aircraft buzzed the island, including crossing the sensitive mid-line of the Taiwan Strait, in an escalation of tensions the same day a senior US official began meetings in Taipei.

Earlier on Friday, China’s Defence Ministry announced the start of combat drills near the Taiwan Strait, denouncing what it called collusion between the Chinese-claimed island and the United States.

U 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.

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 mass air and sea drills last week.

Taiwan’s defence ministry said 18 Chinese aircraft were involved on Friday, a far larger number than Taiwan has previously announced for such encounters.

“Sept 18, two H-6 bombers, eight J-16 fighters, four J-10 fighters and four J-11 fighters crossed the midline of the Taiwan Strait and entered Taiwan’s southwest ADIZ,” the ministry said in an English-language statement on Twitter.

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

The ministry showed a map of the flight paths of the Chinese jets and their crossing of the Taiwan Strait mid-line, which normally combat aircraft from both sides avoid passing through.

Taiwan’s Liberty Times newspaper said Taiwan air force jets scrambled 17 times on Friday morning over four hours, warning China’s air force to stay away.

It also showed a picture of missiles being loaded onto an F-16 at the Hualien air base on Taiwan’s east coast.

In Beijing, Chinese Defence Ministry spokesman Ren Guoqiang said Friday’s drills, 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.

Taiwan is a purely internal Chinese affair that brooks no foreign interference, he added.

“Recently the United States and Democratic Progressive Party (DPP) authorities have stepped up their collusion, frequently creating disturbances,” Ren said, referring to Taiwan’s ruling party.

Trying to “use Taiwan to control China” or “rely on foreigners to build oneself up” is wishful thinking and doomed to be a dead end, he added.

“Those who play with fire will get burnt,” he said.

Hu Xijin, editor of China’s widely read state-backed Global Times tabloid, wrote on his Weibo microblog that the drills were preparation for an attack on Taiwan should the need arise, and that they were valuable experience, enabling gathering of intelligence about Taiwan’s defensive systems.

“If the US secretary of state or defence secretary visits Taiwan, People’s Liberation Army fighters should fly over Taiwan island, and directly exercise in the skies above it,” he added.

China had threatened to make a “necessary response” to Krach’s trip, straining already poor ties between Beijing and both Taipei and Washington. Sino-US relations have plummeted ahead of November’s US presidential election.

Chinese fighter jets briefly crossed the mid-line of the Taiwan Strait last month as the US Health Secretary Alex Azar was in Taipei, and last week China carried out two days of large-scale drills off Taiwan’s southwestern coast.

The United States, like most countries, only has official ties with China, not Taiwan, though Washington is the island’s main arms supplier and most important international backer.

This week, the US ambassador to the United Nations had lunch with Taiwan’s top envoy in New York. China’s UN mission said it had lodged “stern representations” over the meeting.