Seven people have died and up to 21 are missing after a sightseeing boat capsized in the Hungarian capital of Budapest.

The double-decker boat was moored near the Hungarian parliament building on the Danube River when it was hit by another tourist vessel.

State news agency MTI said the boat was carrying 33 South Korean tourists and two crew members.

The rescue effort has been hampered by heavy rains and strong currents on the river.

A fire brigade rescue boat takes part in the search for survivors on the River Danube in downtown Budapest, Hungary, 30-05-2019. Image: Zoltan Mathe/AP/Press Association Images

Seven people have been pulled from the water and are suffering from hypothermia; however, they are said to be a in a stable condition.

One person was rescued over 3km from where the boat sank.

Temperatures in the water were only about 10C to 12C .

A rescue boat searches for victims late after a tourist boat crashed with a river cruise boat on the River Danube in Budapest, 29-05-2019. Image: Peter Lakatos/AP/Press Association Images

Gabor Csato, a spokesperson for the country's ambulance service said rescue operations are ongoing.

“We are looking for passengers on water and ashore,” he said. “We found some people with serious injuries in a critical condition and some needed to be resuscitated.”

More than a dozen emergency vehicles joined fire boats, divers and military personnel in the search. Boats docked along the river bank shine spotlights in the heavy rains.

All shipping on the Danube in central Budapest was suspended as the search continued.

The boat that sank was 89ft long and had capacity for 60 people on two decks.