A lone fishing vessel has strayed near a protected marine sanctuary in Philippine waters, prompting authorities to scramble a navy ship and a surveillance plane to Sulu Sea far south of the country.

Marine rangers saw the small vessel near the World Heritage-listed Tubbataha Reef late Saturday and sought the Philippine Navy’s help after a failed chase, reports AFP.

“We suspect it was Vietnamese but we didn’t catch anyone,” the marine park’s superintendent Angelique Songco said.

“It was dark and they turned off their lights. They had one light on the rear but when they saw our boat coming, they turned it off and ran.”

Songco also claimed the mystery vessel was flying a Philippine flag but the rangers were convinced it was a Vietnamese boat, saying they recognized the vessel’s shape as similar to Vietnam’s fishing fleet.

The navy deployed a patrol gunboat and a surveillance aircraft to the area but failed to find or identify the vessel, navy spokesman Lieutenant Stanley Gonzaga told AFP.

Presidential spokesman Edwin Lacierda said the coast guard had been ordered to investigate whether it was indeed a Vietnamese vessel.

The incident follows high tensions between Taiwan and the Philippines over the shooting of a Taiwanese fisherman by Philippine coast guards amid a maritime dispute in the South China Sea.