Officials are investigating allegations by some passengers that they did not have life-jackets, says Vice Transport Minister Alejandro Maya.

Rescue workers searched on Monday for as many as 16 people believed to be missing after a tourist boat sank the previous day in a reservoir in north-central Colombia, killing six people.

The cause of the accident was still unclear, officials said. Use of the Penol-Guatape reservoir, a popular site for water-sports and tours, was restricted as the search continued.

The boat, El Almirante, was carrying 170 passengers and three crew members, said Margarita Moncada, the head of disaster relief for Antioquia province.

Authorities had so far tallied 134 people rescued, six dead and 15 or 16 missing, Moncada told reporters, adding that they hoped the number of people rescued would continue to rise. Authorities were also unsure if all the people reported to be on the boat had actually been on board, she added.

Officials initially reported nine people dead and 28 missing, but Moncada said some survivors rescued by private boats were taken to different docks on the reservoir shore and so were not immediately accounted for.

People cry as they wait for more information on tourists who were on a ferry that sank in a reservoir in Guatape, Colombia. | Photo Credit: AP

Videos posted on social media showed motorboats coming to the aid of passengers on the upper decks as the boat rocked from side to side. Survivors told local television they heard a loud noise before the boat began to sink.

It is a long holiday weekend in the Andean country and the reservoir is a well-known destination for families.

Officials were interviewing the captain and investigating allegations by some passengers that they did not have life-jackets, Vice Transport Minister Alejandro Maya told journalists at the scene.

President Juan Manuel Santos visited rescue crews at the reservoir, about an hour's drive from the city of Medellin, on Sunday night.