Seven South Korean tourists died and 21 others were missing after a boat sank in the Danube in Budapest.

Seven South Korean tourists died and 21 others were missing after a sightseeing boat capsized and sank on the Danube in Budapest, Hungarian and South Korean officials said Thursday.

The accident happened near the parliament building in the heart of the Hungarian capital after a collision with a larger river cruise boat during torrential rain around 09:15 pm on Wednesday, according to officials.

A total of 33 South Koreans were on board, Seoul's foreign ministry said, confirming the seven dead were Korean.

The youngest was a six-year-old girl, travel agency officials said.

The 26-metre tourist boat, called the "Mermaid," was also carrying two Hungarian crew members.

"Our services have recorded the death of seven people," Pal Gyorfi, a spokesman for Hungarian emergency services, said early Thursday morning.

"Seven people have been taken to hospital in a stable condition with hypothermia and shock symptoms," Gyorfi added.

"A further 21 people are missing," a Hungarian police spokesman Kristof Gal told AFP.

"Police are searching the river throughout the entire length of the Danube in Hungary south of where the incident took place," he said.

All night search

Local media reported that one of the bodies was found several kilometres south of the collision location, although Gal declined to confirm.

The temperature of the river water is between 10 and 15 degrees, according to local media.

The search for the missing with the help of divers and police shining lights continued through the night, said an AFP photographer at the scene.

A film crew working from a bridge south of the accident site also used reflector lights to help light up the water through the gloom and pouring rain, reported local media.

Heavy rainfall since the beginning of May has led to high water levels and a fast-moving river current, complicating rescue efforts.

The accident happened on a popular part of the Danube river for pleasure trips, from where passengers can view the city and parliament building illuminated at night.

The boat was regularly serviced and had no apparent technical faults, Mihaly Toth, a spokesman for Panorama Deck that owned the vessel, told the Hungarian news agency MTI.

"It was a routine sightseeing trip," said Toth.

"We know nothing about how it happened, the authorities are investigating, all we know is that it sank quickly," he said.

Hit by bigger cruise boat

An eye-witness told the Index.hu news-site that the Mermaid, which could hold 60 people on board, had been hit from behind by a large cruise boat.

Web camera footage from a hotel rooftop posted on local news-sites appeared to show the bigger boat colliding with the Mermaid.

The wreckage of the Mermaid was found on the riverbed after several hours of searching near the Margaret Bridge, one of the main bridges connecting the two parts of the Hungarian capital, local media said.

Access to the river has been blocked by the authorities, according to public television.

South Korea's President Moon Jae-in instructed the government to "deploy all available resources" for the rescue, the presidential office said.

Seoul planned to send a team of 18 officials to assist the authorities in Budapest, South Korea's Yonhap news agency reported.

The foreign ministry said minister Kang Kyung-wha would leave for Budapest later Thursday as head of a government taskforce.

Embassy staff have also been assisting the emergency services in the identification of victims.

The Hungarian interior and health ministers visited the scene and expressed condolences to the families of the victims.