At least nine people have died after a tourist boat carrying 160 people capsized in Colombia.

Around 31 people are still thought to be missing after the boat sank on a reservoir near the city of Medellin.

Dramatic videos on social media show the party boat rocking back and forth as people crawled down from the roof as it sank in a matter of minutes.

Survivors described hearing a loud explosion near the men's toilet that knocked out power a few minutes after the boat began its cruise around the giant lake.

As water flooded on board, pressure built and people were sucked under by the sinking ship.

A flotilla of recreational boats and jet-skis rushed to the scene, pulling people from the boat as it went down and averting an even deadlier tragedy.

"Those on the first and second floors sank immediately," survivor Lorena Salazar told local media. "All we could do was scream and call for help... it was completely chaotic."

Margarita Moncada, the head of the disaster response agency in Antioquia state, said that according to a preliminary report 99 people were rescued and another 40 reached shore on their own.

It is unclear what caused the boat to sink.

Some people who witnessed the tragedy from the shore said the boat appeared to be overloaded but Mr Santos said it was sailing well below capacity.

None of the passengers were wearing life vests. Complicating the search, there was no passenger list.