Taiwan said it tracked four Chinese naval vessels and 57 military aircraft in its surrounding sea and airspace on Monday after Beijing signaled its displeasure at Taipei’s warming ties with the West.

At least 22 fighter jets, two nuclear-capable heavy bombers and three military drones were among the 28 Chinese aircraft that crossed the Taiwan Strait median line and operated inside the Taiwanese air defense identification zone in the 24 hours to 6 a.m. local time, Taiwan‘s defense ministry said, in flight paths that appeared to envelop the island from three sides.

China claims Taiwan as its own, but Taipei rejects the sovereignty claims. The median line has acted as an unofficial barrier between the competing militaries for seven decades, while the ADIZ is an additional buffer—used by Taiwan, China and others in the region—to identify approaching aircraft. Beijing has begun to ignore both in recent years, in behavior the United States says is destabilizing.

In an announcement shortly before midnight on Sunday, the Chinese military, known as the People’s Liberation Army, said it had held a “joint combat-readiness security patrol…