BOGOTA, June 25 (Xinhua) -- At least nine people are dead and 28 others missing after a tour boat with some 170 passengers onboard capsized Sunday afternoon in northwest Colombia.

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.

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.

The scale of the accident has 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.

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.

Video footage of the incident showed that vessels of all sizes were rushing to the site in an attempt to help the victims.

"According to preliminary reports, the boat was at full (passenger) capacity, due to the fact that during long holiday weekends, thousands of tourists visit the town," RCN Radio reported.

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.