The search for the 270 passengers still missing on the sunken South Korean ferry Sewol is continuing, as medical experts reveal that the state of the bodies recovered so far is making it difficult to establish time and cause of death.

The victims, whose bodies were recovered on Friday (18 April), are believed to have died shortly after the passenger ship went down.

It has been more than 72 hours since the 6,825-tonne vessel carrying 476 passengers, mostly school students, capsized and sank off the southern coast.

"It's difficult to tell exactly when they died because calculating the [exact time of death] for bodies found in the water and land are different," an expert with the Crime Scene Investigation (CSI) team, told Seoul's Korea Herald on condition of anonymity.

Noting the recovered bodies had begun to lose rigor mortis following their demise, the investigator said: "The bodies show signs that rigor mortis has disappeared. We clean the bodies found on the scene out of respect for their families. So I suppose when the [victim's] parents see the body, they could think they had been dead for only a few hours."

No one has been found alive since the vessel sank while 29 people have been confirmed dead so far.

As many as 174 passengers have been rescued and 273 others are missing.

"We've yet to get any response from survivors underwater. Divers have continued all-out operations to enter cabins, while pumping air to help them breathe. Instead of dispatching two divers at a time, we are to send up to 10 workers at the same time to speed up the job We are working to install more guide lines," South Korean coast guard official Choi Sang-hwan told reporters.

Experts believe it is technically possible to survive inside the hull for 72 hours if there are enough air pockets, but hopes are fading.