MANILA, Dec. 21 (Xinhua) -- At least four people were killed when a ferry carrying 251 passengers and crew sank off Quezon province in heavy waves on Thursday, but majority of those on board were rescued, authorities said.

In an official statement, the Philippine Coast Guard (PCG) said 240 passengers were rescued several hours after the ferry boat sank five nautical miles off Infanta town in Quezon province, southeast of Manila.

The PCG said seven more passengers remain missing.

The passenger ferry left the Ungos port in Real town around 9 a.m. local time and was en route to Polillo Island when it sank around 11:27 a.m. local time, said the PCG.

The vessel has a gross tonnage of 206.27, net tonnage of 61.88 and capacity of up to 286 passengers.

The cause of the disaster is not known, the PCG said.

"As of press time, the PCG is focusing on the search and rescue operations and will immediately conduct investigations to identify the cause of the incident and to verify possible oil spills," the PCG said.

But a survivor, Donel Jade Mendiola, told a local radio that the ferry was hit by strong wind and huge waves a couple of hours before it sank.

Ferry accidents are not rare in the Philippines, an archipelago of 7,100 islands with a poor record for maritime safety.