At least nine people were dead and 28 others went missing after a tour boat with some 170 passengers onboard capsized Sunday afternoon in northwest Colombia.

Image provided by Colombia's National Police shows rescuers and police officers boarding a boat towards the site where a tour boat capsized in Guatape, Antioquia Department, Colombia, on June 25, 2017.

Officials did not yet say what caused the ship called "Almirante" to go down in the Penol-Guatape reservoir in the tourist town of Guatape, in Colombia's northwest department of Antioquia.

"At the moment we officially have nine people who have been found dead," and "approximately 28 people reported missing," said Margarita Moncada, head of the disaster prevention department of the Antioquia regional government.

She said the boat was carrying 170 people, most of whom were rescued by other boats or escaped by themselves.

Eduardo Rivera, who is in charge of a state-run hospital in the town of Guatape, said another 15 people were injured and recovering at the facility. Colombian air force helicopters were dispatched and stood ready to evacuate the most seriously injured.

The Almirante, a four-deck vessel, was taking visitors on a tour of the reservoir, located 80 km east of Medellin, when it began to tilt sideways, then gradually sank.

Survivors on local television reports said they heard a loud noise before the boat began to sink and that not all of them had life vests.

The ship sank extremely quickly, according to Luis Bernardo Morales, a fire service captain involved in the rescue effort. "It all happened in a few minutes."

Videos circulating on social media showed that the boat rocked from side to side as people crawled down from the upper decks when it began sinking into the water, and vessels of all sizes were rushing to the site in an attempt to help the victims.

Some people who witnessed the tragedy from the nearby shore said the boat appeared to be overloaded but President Juan Manuel Santos said it was sailing below capacity.

"I have been informed that the boat had a capacity greater than the number of people on board, so that it was not due to overloading" that the craft sank, he told reporters while visiting the scene of the sinking.

"Nobody really knows what happened," said Santos, adding that naval officials were brought in to carry out an investigation and divers would continue searching for people all night.

"The way it sank, which was very quick and sudden, raises questions," he added.

Santos also said that "there are no children" among the dead so far.

The scale of the accident led to the mobilization of Colombia's armed forces, Victoria Eugenia Ramirez, secretary of the government of Antioquia, told the media.

"We have a full operation underway. The rescue operation includes the interior ministry, air force, army and police. The boat 'El Almirante' (The Admiral) did not crash into another, it just sank," Ramirez added.

A local vendor told the Caracol News network that it was not the first time the boat has sunk.

"This boat sank three months ago, but at the pier, with no passengers. The owners fixed it and it continued to operate," said Marylin Usme.

Medellin's Mayor Federico Gutierrez announced via Twitter that he was dispatching specialized personnel, including five fire department scuba divers, for the rescue work.

"The area is a bit difficult to reach and various rescue and emergency teams have started joining in," he said on Blu Radio.

The large reservoir, about an hour from the central city of Medellin, is one of Antioquia department's main tourist draws. It was especially busy on Sunday as Colombians celebrated a long holiday weekend.

Guatape is a popular Andean resort town, known for its water sports and recreational activities.

Source: Xinhua