The captain of the ship that collided with a sightseeing boat that sank on on the Danube River, killing at least seven people and injuring many more, has been taken into custody.

The 64-year-old Ukrainian man is suspected of endangering water transport leading to a deadly mass accident on the river in the heart of Budapest, police said in a statement on Thursday.

In line with Hungarian laws, the suspect was identified only as Yuriy C, referring to his first name and the initial letter of his last name.

So far just seven of the 35 people on board – 33 South Koreans and two crew members – are known to have survived Wednesday’s accident near the Hungarian Parliament in Budapest.

Seven others are confirmed dead and 21 remain missing after the tourist boat sank just seven seconds after being hit. Hopes are dimming that any further passengers will be found alive after two nights in the freezing, flooded Danube.

The youngest of the missing is just six years old, the oldest in their 70s. Reportedly, none of the tourists on board the sightseeing vessel, the Mermaid, were wearing life jackets when the accident happened.

BREAKING: The Latest: South Korean tourists in deadly boat collision in Budapest weren't wearing life jackets. https://t.co/a1QZAwf93G

— The Associated Press (@AP) May 30, 2019

Hungarian police said on Friday (AEST) they had detained Yuriy C, an Odessa resident, after questioning him. They have proposed his arrest.

The Mermaid and the second vessel, the Viking Sigyn, collided on Wednesday, before the tourist boat turned over on the river, which has been swollen by heavy rains.

CCTV footage shows the Mermaid being hit from behind by the Viking Sigyn near the Margit Bridge in central Budapest.

“The whole thing happened very quickly” Clay Findley, an American tourist aboard the Viking Sigyn told the BBC.

“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.”

The Viking Sigyn moored in Budapest. Its captain has been arrested. Photo: Getty

The massive rescue effort involves boats, divers, spotlights, and radar scanning to scour the Danube for several kilometres downstream from the accident site.

But there is heavy rain in Budapest and the Danube is already flooding, leading to strong currents that are hindering the rescue.

The chief operating officer of the South Korean tour group, Lee Sang-moo, has told a media conference in Seoul that the group was on the fourth day of a nine-day tour of eastern Europe.

“I deeply apologise to the parties affected by the accident, families of victims and the people who are suffering after hearing the news this morning,” Mr Lee said.

“I understand that the boat took every precaution.”

South Korean President Moon Jae-in has already said South Korea will work with the Hungarian government to investigate the cause of the accident.

-with AAP