Marine rescue authorities in south China's Guangdong province said Monday that seven people, mostly teenagers, on board a wooden boat had been missing for four days in the sea off Zhuhai city.

The provincial marine rescue center said two teams of rescuers had been scouting for the boat since Friday, so far in vain.

A rescue official said families of the missing called police for help on Friday, saying the group did not return home after sailing off to reap clam that morning on board the 10-meter long wooden boat.

The official said the sea area had been enveloped in a dense fog over the weekend. A helicopter was only able to join the search on Monday, but failed to find traces of the boat.

The center has coordinated with local fishing affairs authorities and police to continue the search on sea.