The tourist vessel Viking Sigyn which was involved in a ship collision that killed seven people on the Danube River in Budapest, Hungary. Picture: Bernadett Szabo/Reuters

Budapest - The captain of a boat which collided with a smaller sightseeing boat on the River Danube killing at least seven people has been arrested, police in Budapest said late Thursday.

There were personal and material reasons to suspect the Ukrainian national, police said, without giving any further details.

Meanwhile the search for 21 people still missing after Wednesday's incident in the Hungarian capital continued, although hopes of finding the victims alive were small as rescue work was hampered by strong currents and poor visibility.

The body of one person was found kilometres away from the site of the collision, police said.

The smaller boat, Hableany, had 35 people on board - 33 South Korean tourists and two crew members - when it was hit late Wednesday near the Margaret Bridge and sank within seconds.

The skipper and the other crew member of the Hableany were among those missing, police spokesman Adrian Pal told a news conference in Budapest earlier Thursday. 

The Hableany had turned in front of the larger boat, the Viking Sigyn, before it was hit, Pal said. No one on board the Viking was injured.

Seven survivors were plucked from the river shortly after the accident by passengers on other boats. They had to be treated for hypothermia even after a relatively short time in the water, rescuers said.

Preparations also began Thursday to raise the wreck of the Hableany, with soldiers building a special diving platform, although conditions were hampering their work as well.

"The raising of the wreck could take days, even a week," state-run news agency MTI quoted the chief of a company, which specializes in raising wrecks, as saying.

Meanwhile the first relatives of the South Korean victims set off for Hungary, according to South Korean broadcasters.

Ten relatives had already set off and another 30 were set to follow later on Friday. Foreign Minister Kang Kyung Wha was also expected in Budapest as well as South Korean rescue workers, including specialist divers.

In Seoul, Vice Foreign Minister Lee Tae Ho expressed disappointment with the slow pace of the rescue work.

"It is very regrettable that there has not been any change [for the better] as to the situation," he said during a government disaster management session, according to the news agency Yonhap.

"There has been no progress in rescue operations due to strong currents of waters there on top of bad weather conditions."