TAIPEI (Taiwan News) — The Ministry of National Defense said it had tracked 20 aircraft and four ships from China’s military around the country by 5 p.m. Friday (Sept. 23).

Four of the aircraft entered the southwest sector of Taiwan’s air defense identification zone (ADIZ), the ministry tweeted. They included two Shenyang J-16 fighter jets, but also two reconnaissance drones, a Guizhou BZK-007 unmanned aerial vehicle and a Guizhou WZ-7 Soaring Dragon.

While fighter jets and anti-submarine aircraft have made frequent inroads into Taiwan’s ADIZ for more than a year, incursions by drones have been a more recent phenomenon, occurring mostly since United States House Speaker Nancy Pelosi’s Aug. 2-3 visit to Taipei. Taiwan’s military shot down a drone from China near the island of Kinmen on Sept. 1.

On Friday, the military tasked aircraft, naval vessels, and land-based air defense missile systems to monitor and respond to the Chinese activities.