The unfolding tragedy was compounded by the apparent suicide of a high school vice principal who had been rescued from the 6,825-tonne Sewol that sank Wednesday morning with hundreds of his students trapped inside.
More than 48 hours after the ferry suddenly listed and capsized, exhausted dive teams -- who waded through powerful currents -- finally managed to access the interior.
After several attempts, two divers managed to pry open a door and enter the cargo section, a senior coastguard official said, briefing relatives of the missing.
"The search operation will continue through the night," the official said.
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"Visibility is almost non-existent. You can hardly see your hand in front of you face," said one diver when he returned to the harbour at nearby Jindo island.
The confirmed death toll stood at 28, but the focus of concern remained on the 268 still unaccounted for.
Of the 475 people on board when the Sewol capsized, only 179 were rescued and no new survivors have been found since Wednesday.
Three giant, floating cranes reached the rescue site, but regional coastguard commander Kim Soo-Hyun stressed they would not begin lifting the multi-deck ferry until they were sure there were no survivors inside.
More than 350 of those on board were from the Danwon High School in Ansan city just south of Seoul.
Among the parents of those students still missing there was bitter resentment at what they saw as the inadequacy of the official response.
"It's been two days but no one has been brought out alive," complained Lee Yong-Gi, whose son was among the missing students.
"I firmly believe that the kids are alive. We need to rescue them as soon as possible. But officials are dragging their feet," Lee told AFP.