JINDO, South Korea, April 24 (Xinhua) -- Death toll from a sunken South Korean ferry rose to 171 on Thursday, but bodies recovered from the ship grew at a slower pace as search operations were hampered by unexpectedly swift currents.

On the ninth day since the ferry carrying 476 people capsized off the country's southwestern coast on April 16, 12 more bodies were retrieved from the submerged vessel, raising the death toll to 171, with 131 still missing and those rescued remaining unchanged at 174.

On Wednesday alone, 38 bodies were recovered from the submerged hull, with 36 found Tuesday and 28 others retrieved Monday.

Rescue operations were suspended for around six hours in the afternoon as tidal currents became faster than expected.

Meanwhile, some 40 family members of the victims protested against the sluggish rescue operations at the office of the government disaster response headquarters.

Weather forecast showed that tidal currents off Jindo Island, where the 6,825-ton ferry Sewol sank, would slower for four days from Monday.

Divers have been searching bodies or possible survivors believed to be trapped in passenger cabins on the third and fourth floors of the five-storey ship.

Koh Myung-seok, spokesman of the disaster response headquarters, told reporters that it was very tough for divers to break into the cabins, saying divers had to clear up junks, including beds and other furniture, in every cabin.

Among three blocks of the vessel, search for cabins on the starboard side were completed Wednesday, and divers were focusing on the middle of the ship. Cabins on the port side had yet to be reached.

Divers became fatigued due to the protracted underwater operations. One navy diver got decompression sickness while diving for a long time.

Around 700 divers are conducting search operations in the tough and murky waters, the coast guard said.

In addition, all surviving sailors are widely expected to receive criminal penalties as four of 11 arrested crew members have already faced criminal charges.

A special investigation team arrested four more sailors Thursday after taking seven sailors into custody.

The 69-year-old captain was arrested on April 19 on five charges including negligence of duty along with the third mate who steered the ship when the accident occurred.

Four more crew members, including two first mates, one second mate and one chief engineer, were arrested Monday for their role in the deadly incident.

The remaining four sailors were widely expected to be arrested soon.

The 15 surviving sailors have been criticized for fleeing the sinking vessel without making efforts to save passengers.

Prosecutors questioned eight officials of the company responsible for safety checks of the submerged ferry.

Prosecutors on Wednesday raided the office of the sunken ferry operator, Cheonghaejin Marine, under suspicion that it had illegally modified the ferry to expand passenger cabins and overloaded cargo.