A Philippine oil tanker sank in Manila Bay early Thursday after encountering huge waves, and the coast guard was working to determine whether the vessel was leaking oil — in what could be a major spill — after it rescued 16 of 17 crew members in a nighttime operation, officials said.

According to the Associated Press (AP), the tanker Terra Nova left Bataan province en route to the central province of Iloilo with about 1.4 million litres (370,000 gallons) of industrial fuel oil stored in watertight tanks when it got buffeted by huge waves and took on water.

The crew struggled to steer the tanker back to port but it eventually sank shortly after midnight, coast guard spokesperson Rear Admiral Armando Balilo said, citing statements from surviving crew members.

The sinking followed days of monsoon rains, exacerbated by a passing offshore typhoon, that caused landslides and floodings across the archipelago, leaving at least 22 people dead and displacing more than half a million people.