Rapid river currents are hindering rescue efforts in the Hungarian capital Budapest days after a tour boat sank.
At least seven people died and more than 20 remain missing after a large vessel collided with the Hableany, or Mermaid, near the the Margit (Margaret) Bridge on the Danube on Wednesday.
Relatives of South Korean victims have been arriving in Hungary.
South Korea’s foreign minister has vowed not to give up the search for more survivors.
Kang Kyung-wha, speaking at a press conference with Hungary’s foreign minister Péter Szijjártó, said she and her team “won’t give up our hopes about the possibility of finding survivors”.
“We strive to ensure no corpses are lost from the wreckage or the riverbed,” she added.
But officials say the conditions on the river are stopping several hundred rescuers on the scene from reaching the sunken ship.
“We have to say that circumstances are working against us,” Mr Szijjártó said.
Water levels, inflated by heavy rain, are not predicted to drop until Tuesday. The search has been expanded to cover the whole length of the river, and Hungary has contacted Serbian authorities downstream.
Thirty South Korean tourists and three tour guides, as well as two Hungarian crew, were on board the Hableany.
Only seven people are confirmed to have survived the incident, while seven South Korean tourists are known to have died.
Most of the tourists were aged between 40 and 50 but the group also included a six-year-old child and a man in his 70s, South Korea’s Yonhap news agency reported.
What happened to the tour ship?
CCTV footage showed a larger tour boat – Viking Sigyn – hitting the Hableany from behind in central Budapest after 21:00 local time (19:00 GMT) on Wednesday.
“The whole thing happened very quickly” said Clay Findley, a US tourist who was on the Viking Sigyn.
“I thought at first we were going to miss it, but the front of the Viking hit the back of that little boat… and then the hull popped up on the opposite side of the ship, just a few seconds later, and then it was down.”
Viking Sgyn’s captain, identified as 64-year-old Ukrainian national Yuriy C, has been held as a suspect over reckless misconduct in waterborne traffic leading to mass casualties.
His lawyers issued a statement – carried on state news agency MTI – saying the captain denies breaking any rules or laws.
He was “shaken by the consequences of the accident”, they said, and expressed his condolences to the victims’ families.
How are rescue efforts going?
Emergency crews found the wreckage of the Hableany, a double-decker river cruise boat built in 1949 in the former Soviet Union, on the riverbed near the Margaret Bridge and were preparing to lift it.
A floating crane has been set up near the ship but an interior ministry statement said that the rapid currents “preclude any approach of the hull”.
One diver was immediately swept away in the Danube, and had himself to be be rescued.
For South Koreans, the sinking is a painful reminder of the Sewol disaster in 2014, the BBC’s Laura Bicker in Seoul reports.
The ferry of that name sank off South Korea’s Jindo island killing 304 people, almost all of them schoolchildren on a trip. The ship’s captain was later convicted of murder.