TAIPEI--Hong Kong air traffic controllers on Thursday warned off a Taiwanese civilian flight flying to Taiwan-controlled islands in the South China Sea, forcing it to turn back, the island’s government said.

The incident, which comes at a time of rising regional tensions, occurred as a UNI Air charter flight from southern Taiwan’s Kaohsiung to the Pratas Islands was approaching Hong Kong’s flight information region, Taiwan’s Civil Aeronautics Administration said.

Hong Kong air traffic controllers told the captain there were “dangerous activities” happening below 26,000 feet and the aircraft could not enter, so for safety reasons he decided to turn back, the administration said in a statement.

Hong Kong had not issued a prior notice warning of activities that could affect flights, it added.

Hong Kong’s Civil Aviation Department said they had told Taiwan air traffic controllers of the minimum safe altitude to be observed for the flight, and that Taiwan had then responded by saying they had cancelled the request to enter the Hong Kong flight information region.

“During the incident, Hong Kong’s Civil Aviation Department all along acted in accordance with established mechanisms and procedures,” it added.

The generally weekly charter flights to the Pratas mostly carry Taiwan government and coastguard personnel and are not open for ordinary travelers to book.

The Pratas lie southeast of Hong Kong at the northern end of the South China Sea, and normally get far less attention than the more contested waters further south near the Paracel and Spratly Islands.

China, which claims democratic Taiwan as its own territory, has stepped up military activities near Taiwan, including drills and regular air force missions close to the Pratas, which are only lightly defended by Taiwan.

The Pratas, the closest Taiwan-controlled territory to Hong Kong, have taken on extra significance since anti-government protests began in the Chinese-run city.

Taiwan has intercepted at least one boat close to the Pratas carrying people fleeing from Hong Kong trying to make their way to Taiwan.

Some Taiwanese officials have expressed concerns that China could seize the Pratas, in a drastic escalation of tensions that could lead to a war.