The U.S. military flew three Air Force planes — including two B-52 bombers — near Taiwan on Wednesday morning, the country’s Defense Ministry and the U.S. Pacific Air Forces said, after China sent its own bombers and fighter jets for exercises near the island on Sunday and Monday.

The ministry said in a statement on its website Wednesday that a U.S. MC-130J special mission aircraft flew across the Taiwan Strait from north to south, while two B-52 bombers flew along Taiwan’s east coast, also from north to south. The ministry said it was aware of the flights and had monitored them throughout.

The U.S. Pacific Air Forces also confirmed that two B-52s had flown from Andersen Air Force Base on the U.S. territory of Guam and conducted “synchronized training south of the Taiwan Strait with a U.S. Air Force MC-130J” from Kadena Air Base in Okinawa.

The U.S. said its aircraft “regularly operate throughout the region in support of allies, partners and in defense of a free and open Indo-Pacific,” but the statement by Taiwan’s Defense Ministry was noteworthy in that such flights are typically not announced by Taipei.

The U.S. has no formal ties with Taipei but is bound by its Taiwan Relations Act to help it defend itself, and Washington is the island’s main source of arms.

Wednesday’s flights appeared to be a response to the Chinese drills earlier in the week.

On Monday, Taiwan’s military scrambled fighter jets to intercept Chinese aircraft that briefly crossed an unofficial median line of the strait that separates the two sides, according to the ministry.

The Chinese Defense Ministry said later in the day that the naval and air forces of the military’s Eastern Theater Command had conducted a joint drill in waters off the southeast coast of Taiwan. Those drills “focused on real-combat subjects such as air-to-ground assault and fire support and further honed the multiple services’ capability in joint operations,” senior Col. Zhang Chunhui, spokesperson for the Eastern Theater Command said.

The Chinese planes flew over Bashi Channel, the waterway connecting the South China Sea with the western Pacific Ocean, as part of long-distance training missions, and returned to their bases the same way.

On Sunday, Taiwan’s Air Force also scrambled fighters when Chinese jets and bombers flew through the Bashi Channel to the south of Taiwan and into the western Pacific Ocean before returning via the Miyako Strait, the ministry said.

Beijing has called Taiwan “the most important and sensitive issue in China-U.S. relations” and has bolstered its military presence near the island, sailing its new aircraft carrier through the Taiwan Strait in December and holding large-scale “encirclement” exercises and bomber training around the island since 2016, when Taiwan President Tsai Ing-wen first took office.

China has grown suspicious of Taiwanese President Tsai Ing-wen and her pro-independence Democratic Progressive Party, along with any push for the island’s formal independence.

Zhang, the Chinese military’s Eastern Theater Command spokesman, on Tuesday lambasted “Taiwan independence,” and said Beijing would be on the lookout for continued “independence” efforts.