China on Thursday condemned the U.S. military for the “provocative” flight of one of its aircraft over Chinese-claimed Taiwan, saying the move infringed upon China’s sovereignty and contravened international law.

China considers democratically ruled Taiwan its own territory and one of its most sensitive diplomatic issues, regularly denouncing the United States for its support of the island.

A U.S. C-40A, a military version of the Boeing 737, entered Taiwan air space with permission, though it did not land at any Taiwan airports, Taiwan’s Defence Ministry said on Tuesday.

The U.S. aircraft took off from Japan’s Okinawa island, where there is a major U.S. air base, and flew over northern and western Taiwan on its way to Southeast Asia, Taiwan media reported.

China’s Taiwan Affairs Office said the U.S. aircraft had “harmed our sovereignty, security and development rights, and contravened international law and the basic norms of international relations”.

“It was an illegal act and a seriously provocative incident,” the office said in a statement carried by state media. “We express strong dissatisfaction and resolute opposition.”

The U.S. Air Force did not immediately respond to a request for comment.

Taiwan is entirely separately governed from China and controls its own air space.

On the same day as the U.S. aircraft flew over the island, Taiwan’s air force had to warn off several Chinese fighter jets which briefly entered Taiwan’s air defence identification zone. Taiwan has repeatedly complained about Chinese drills near the island.

While Washington and Taipei have no formal diplomatic ties, the United States is Taiwan’s strongest international supporter and main arms supplier, becoming another source of U.S.-China tension.

The United States has stepped up its military activities near the island too, with semi-regular U.S. Navy voyages through the narrow Taiwan Strait that separates the island from China.

Reporting by Beijing newsroom; Additional reporting and writing by Ben Blanchard in Taipei; Editing by Nick Macfie