Hundreds of rescue workers try to help at the bank of Danube River at Budapest downtown. (GERGELY BESENYEI/AFP)

The boat was cruising on the river when it collided with a larger luxury passenger boat during a rainstorm on Wednesday evening, causing it to capsize and sink with 33 South Korean tourists and tour guides onboard and two Hungarian crew.

"I wouldn't say there is no hope, rather that there is a minimal chance (of finding survivors)," Pal Gyorfi, a spokesman for the Hungarian national ambulance service, told the M1 state broadcaster.

"This is not just because of the water temperature, but (also) the strong currents in the river, the vapour above the water surface, as well as the clothes worn by the people who fell in," he added.

Police said the smaller boat turned on its side and sank within seconds of the collision under a bridge near Hungary's parliament building.

They said a criminal investigation was under way to determine the cause of the accident. Police declined to say if the bigger vessel, the 135-metre (443 ft) Viking Sigyn, put out any signals for help.

Heavy rains have left the Danube in spate, and local flooding and strong currents hampered rescuers, one per told M1. The water temperature in the river was 10 to 12 degrees Celsius, he said.

Officials said the hull of the Mermaid, a 27-metre (89-ft) double-decker river cruise boat, had been found on the riverbed a few hundred metres from its usual mooring point.

A crane ship docked near the wreck early on Thursday in preparation for recovery operations.

Rescue boat on the Danube river after a tourist boat capsized in Hungary on May 29, 2019. (Photo: Reuters/Bernadett Szabo)

Hungarian Prime Minister Viktor Orban offered his condolences to Seoul. South Korean President Moon Jae-in said the authorities would work with the Hungarian government to investigate the cause of the accident.

"What's most important is speed," Moon said in Seoul.

SEVEN SECONDS

The foreign ministry said 30 South Korean tourists, including at least one child, three South Korean tour guides and two Hungarian crew were on board when the accident occurred at around 9pm (1900 GMT).

Seven South Koreans were rescued, seven died, and 19 South Koreans were among the 21 missing, said Kang Hyung-shik, a foreign ministry official. The crew members were also missing.

The seven people rescued were suffering from hypothermia but stable, a Hungarian ambulance spokesman said. Police said the seven people who died had no life vests on.

"We have found the wreckage with a sonar," police Col. Adrian Pal told a news briefing. "We are looking at ways to extract (the Mermaid) from the river."

"At this point we cannot confirm or deny whether there are any bodies trapped inside the hull."

Video footage from a security camera screened by police showed the bigger boat catching up from behind at a higher speed, clipping the Mermaid's left side.

"The (Mermaid) for some reason, turns into the way of the Viking. And the Viking as it bumps into it, pushes it ... and within seven seconds ... (Mermaid) sinks," Pal said, describing the video.

RESCUE EFFORT

Rescue officials deployed boats, pers, floodlights and radar, scouring the river for several kilometres (miles) downstream from the site of the accident.

A rescue boat on the Danube river after a tourist boat capsized in Hungary on May 29, 2019. (Photo: Reuuters/Bernadett Szabo)

The ambulance service extended its search for survivors and casualties downstream from the capital Budapest and was on alert along a section further south.

For a while traffic on the river, an important commercial waterway, was suspended on the entire Hungarian stretch of the Danube south of Budapest.

The Mermaid's owner said the boat - a Soviet model manufactured in 1949 and refurbished in the 1980s - had been in its fleet since 2003, with regular maintenance.

"We are mobilising every resource we have to protect human lives," the owner Panorama Deck Ltd. told state media through a spokesman.

The larger Viking Sigyn is a 95-room floating hotel of the kind that has multiplied as Danube river cruises gained popularity in recent years.

"There were no injuries to Viking crew or Viking guests. We are cooperating with the authorities as required," said a spokesperson for operator Swiss-based Viking Cruises Ltd./.

VNF/Reuters