Two United States military aircraft reportedly flew close to Taipei’s southern airspace on Monday, the Taipei Times said.

The newspaper, citing flight charts posted on Twitter, identified a U.S. RC-135W Rivet Joint and a Lockheed P-3 Orion in the airspace north of the South China Sea. Aircraft Spots, a military air movement tracker, posted the images.

The two planes flew at different intervals, with the RC-135W reconnaissance plane spotted first over the Bashi Channel, southwest of Taiwan, according to the flight charts. The Bashi Channel connects the South China Sea with the western Pacific Ocean and runs between Taiwan and the Philippines. China claims nearly the entire South China Sea despite an international court ruling in 2016 that its claims are entirely illegal.

Taiwan’s Ministry of National Defense (MND) did not directly confirm the aircraft sightings. However, MND spokesman Major General Shih Shun-wen said Taiwan’s military is closely monitoring the nation’s surrounding waters and airspace.

The U.S. Air Force presence near Taiwan’s airspace on Monday comes as a direct response to China staging military drills near Taiwan over the weekend.

On Saturday night, Taiwan scrambled warships to monitor the movements of a Chinese People’s Liberation Army [PLA] carrier strike group, which was spotted passing through the Miyako Strait near Okinawa by Japan. The Miyako Strait is situated roughly 200 miles east of the northernmost tip of Taiwan. China’s first aircraft carrier, the Liaoning, led the fleet.

The U.S. responded to this Chinese aggression soon after, showing support for Taipei by flying military aircraft near Taiwanese airspace later that same day.

“This is probably the main reason U.S. reconnaissance planes have recently been spotted operating near the Bashi Channel, to monitor the movements of the [PLA] in the area,” Su Tzu-yun, a senior analyst at the Taipei-based Institute of National Defense and Security Research, told the Taipei Times on Sunday.

Last Friday, China’s PLA conducted military drills featuring warplanes near Taiwan. At the time, Chinese state media said the action was part of “military struggle preparations against the island.” Over the past few months, Beijing has increased its aggression towards Taipei, with the PLA ramping up its activities around the independent island.

China views Taiwan as a breakaway province, despite the fact that Taipei has never been governed by Beijing in its history. Though it claims to be Taiwan’s ally, the U.S. has yet to formally recognize the island nation as a sovereign state.

On Sunday, a spokesman for Taiwan’s Presidential Office, Ting Yun-kung, assured the public that there was no need to worry about the nation’s safety and security.