Oxygen has been pumped into the ship to keep it afloat and to help any survivors who could be trapped inside, South Korean officials were quoted as saying.
Underwater robots were also reportedly being made available to try and locate any survivors.
Three salvage cranes arrived at the scene early Friday to either move the 35 meter-deep hull to an area where the tidal current was weaker or to salvage the vessel.
- Risk of death
Experts say people can survive for 72 hours in cold water if there are air pockets in any of the ship's compartments.
However, the extremely cold water temperature has increased the risk of people freezing to death.
The ferry was traveling from the northwestern port of Incheon to the southern resort island of Jeju when it sank off South Korea's southern coast.
It has apparently careened off course and radioed for help while two kilometers off Byungpoong island.
At least 325 of the 475 passengers on board were reportedly high school students on their way to Jeju for a four-day school trip.
Of the students, 69 have so far been rescued and treated.
Search and rescue operations were suspended due to bad weather on Thursday as military divers trying to get inside the capsized vessel were hampered by poor underwater visibility and strong currents.
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