Two Chinese sailors have been rescued more than 50 hours after their sand dredger capsized off Malaysia's southern coast.

Divers heard knocking sounds and found the two sailors in the ship's engine room on Friday.

Coastguard official Sanifah Yusof said they were trapped in an air pocket that had kept them alive after the vessel overturned on Wednesday.

He said they were weak and dehydrated and are now being treated in hospital.

Aerial photo provided by Malaysian Maritime Enforcement Agency shows the capsized sand dredger in the waters off southern Malaysian state of Johor, 22-03-2018. Image:  MMEA/Xinhua News Agency/PA Images

The rescue has fuelled hopes that others may still be alive and Mr Sanifah said that air is being pumped into the ship to help anyone who may still be in there.

Four people are known to have died and nine others are still missing after the dredger went down near the state of Johor.

Rescue teams discovered a body floating near the vessel on Saturday, and two more inside the ship.

A team of Chinese divers joined the search operation and rescue efforts have been intensified.

The submerged vessel has been moved closer to shore and stabilised to stop it from sinking.