BUDAPEST — Rescue divers and boats using spotlights and radar scanners made a last-ditch attempt to find survivors Thursday after a sightseeing boat sank on the Danube in the Hungarian capital during an evening downpour. Seven people were confirmed dead, and 21 were missing.

The dead and 19 of the missing were South Korean tourists who were not wearing life jackets, officials said. The boat collided with a larger cruise ship during the rainstorm, overturned and sank, police reported.

A video presented by police showed the Hableany (Mermaid), a 89-foot double-decker cruise boat, heading north next to the much larger Sigyn, which belongs to the cruise line operator Viking, when the two collided Wednesday night.

Police said the smaller vessel swerved into the larger one, turned on its side and then sank in seconds. They have opened a criminal investigation of the incident, which occurred just after 9 p.m. in the heart of Budapest near the ornate parliament building.

Viking said that no one on its ship was injured and that it is cooperating with authorities. The company, which has offices in Switzerland and California, provided no further details. The ship is now anchored nearby.

Seven people were rescued from the water and have been hospitalized, said Pal Gyorfi, a spokesman for Hungary’s National Ambulance Service, according to the Associated Press. They were treated for hypothermia, and all but one had been released by Thursday afternoon.

Gyorfi expressed doubt that many more survivors would be found. Water temperatures were 50 to 54 degrees Fahrenheit.

Panorama Deck, the company operating the Hableany, expressed condolences and said it has offered financial aid to survivors and victims’ families. It said the boat had been in its service since 2003 and that its 58-year-old captain was an experienced sailor.

The search and recovery operations are hampered by the Danube’s swift current. Days of rain have left the river near flood levels, making it too dangerous for divers to look for bodies in the wreck, police said in a statement Thursday evening.

Richard Sopronyi, a professional diver, told the MTI state news agency that the Danube already has terrible visibility and that the fast current would be life-threatening, so the divers would not be allowed inside the wreck.

Instead, the boat will have to be raised from the river floor before any bodies inside can be recovered, police said.

With currents swirling and the river flowing fast, rescuers also were searching several miles downstream from the wreck.

South Korea’s government said it was sending 33 emergency rescue workers, military experts and officials to Hungary to help with the rescue and recovery efforts.

“What’s most important is speed,” President Moon Jae-in said in an emergency government meeting. He instructed officials to use “all available resources to help.”

Officials in Seoul confirmed that seven South Korean tourists are dead and 19 are missing. Two Hungarian crew members also are listed as missing.

Seoul’s Foreign Ministry and the Very Good Tour Agency, which organized the trip, said 30 South Korean tourists, two tour guides and a photographer were aboard the boat. 

Most of the tourists were traveling with their families, and the group included a 6-year-old girl whose name did not appear on the list of survivors provided by the tour agency, the AP reported. Six women and one man between the ages of 31 and 66 survived, the news agency said.

The National Directorate General for Disaster Management said that it will be a complex operation to raise the boat from where it sank near the Margaret Bridge, which connects the two halves of the capital — the twin cities of Buda and Pest — and that it probably will involve the construction of a pontoon bridge.

The Margaret Bridge has been closed to pedestrian traffic.

The popularity of cruises through the historic heart of Budapest in recent years has resulted in increasing numbers of boats clogging the Danube, raising concerns about possible collisions.

Budapest Mayor Istvan Tarlos, who is seeking reelection in the fall, told the Index online news portal that he was in talks with the relevant ministry about the rules governing these boats and whether they should be allowed to anchor in the downtown area.

Denyer reported from Tokyo.