Hungarian police have arrested the captain of a Viking River Cruises ship after it collided with a small tourist ship on the Danube, killing at least seven South Korean tourists.

There are 21 other passengers unaccounted for, and fears the death toll will rise dramatically once divers reach the tourist boat, which is at the bottom of the river.

According to news wires, relatives of the South Korean passengers are flying in to Budapest, where the accident happened.

Video shows the Viking Sigyn, a 95-suite ship, was involved in a collision with the small sightseeing boat, the Mermaid as it was approaching the Margaret Bridge in the heart of Hungary’s capital around 9pm on Wednesday night.

The Mermaid was carrying 33 South Korean passengers and two Hungarian crew when the Viking Sigyn came into contact with the ship. The Mermaid capsized and sank in seven seconds, according to authorities.

At least seven people have been confirmed dead and emergency crews were able to rescue another seven people from the water.

Rescue workers are still searching the river in freezing conditions and divers hope to reach the sunken sightseeing ship today. One man was found alive three kilometres downstream last night.

The 64-year-old Ukrainian captain of the Sigyn is currently in custody. He is being questioned over endangering water transport leading to a deadly mass accident, police said in a statement on their website on Thursday.

In line with Hungarian laws, the suspect was identified only as Yuriy C, referring to his first name and the initial letter of his last name.

Police were reported as saying they detained the Odessa resident after questioning him and have proposed his arrest.

“Both ships were heading north… and when they arrived between two pillars of the Margaret Bridge, for some reason the Hableany (Mermaid) turned in front of the Viking ship.

“As the Viking comes into contact with it, it overturns and in about seven seconds, as it turned on its side, it sank,” Col Zsolt Gabor Palotai, head of Budapest’s disaster management office, told reporters.

Video footage showed that the two vessels were traveling closely side by side as they approached the bridge in central Budapest. It was not immediately clear what caused the collision.

Police Col. Adrian Pal told reporters “Several people fell into the water from the ‘Hableany’.”

Police said none of the bodies were found wearing life jackets.

Col Pal said search operations along the Danube had been extended downstream into neighbouring Serbia and Hungarian.

A survivor identified as Chung (31) told South Korea’s Yonhap news agency: “They were struggling to stay afloat and shouting for help, but I was unable to do anything. As they were being swept away in fast currents, there was no help coming to the scene.”

A woman named Lim (82) told the country’s JoongAng Ilbo newspaper: “My son-in-law and my daughter are among the missing . . . they went with other couples whom they know through their high school alumni group. I wished them well on their trip, but I didn’t know that was to be our last goodbye.”

Ginger Brinton (66), a US tourist on the Sigyn, told the AFP news agency: “We were on our balcony, and we saw people in the water, screaming for help. We never felt any bump . . . It was just terrible.”

CCTV footage recorded on Wednesday night showed that the river was busy with boats of different sizes traveling in both directions.

Viking said in a statement:  “The Viking river ship Sigyn was involved in a collision with another vessel – a Hungarian tourist ship – on the Danube River in Budapest yesterday evening (29th May).

“Sadly, there have been reports of fatalities and we offer our heartfelt condolences to those affected by this tragic accident. There were no injuries to Viking crew or Viking guests. We have been and continue to cooperate fully with the authorities while they undertake their investigations.”

Officials in Seoul say seven South Korean tourists are dead and 19 others are missing after their sightseeing boat collided with a larger cruise ship and capsized. Two Hungarian crew are also missing.

According to reports, video footage shows the 442ft Viking Sigyn colliding with the 89ft Mermaid close to the Parliament building amid torrential rain.

According to the Daily Mail, the Viking appeared to collide with the Mermaid from behind, before the smaller Soviet-era vessel, spun sideways on impact.

The Mermaid then capsized before sinking in just seven seconds on a fast-moving stretch of the river which has swollen during heavy downpours throughout May.

Hungarian Prime Minister Viktor Orban and President Janos Ader sent condolences to Seoul. ‘We are with the victims and their families in our thoughts and prayers in these most difficult times,’ Mr Orban said, according to his spokesman.

Police were searching the river throughout the entire length of the Danube in Hungary south of where the incident took place.

The temperature of the river water is between 10 and 15 degrees.

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.

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

Most of the Koreans were family groups, and they included a six-year-old girl. Her status wasn’t immediately clear.