Nine people are dead and 28 still missing after a sinking tourist boat carrying 150 in Colombia's north west sparked a rescue effort involving the Colombian air force and local fire services.

"Our aircraft and crews are ready and ready to evacuate the most serious people and require immediate attention," the country's air force said in social media posts.

The military wing also posted pictures of helicopters they said were heading "to Guatape in support of emergency of sinking of boat with around 150 people."

Guatapé is a town in the South American country's north west, just east of Medellin and is known as as a tourist spot.

Medellin's mayor, Federico Gutiérrez‏, said on Twitter that the city sent a fire crew to help support rescue efforts.

The Associated Press also reported that Gutierrez had asked motorists to stay off the roads so as not to hold up rescue efforts. "The most important thing right now is to save lives," he said.

Pictures on social media appeared to show a multi-level ferry, with 'El Almirante' written on the side, sinking into the water. 'El Almirante' is Spanish for 'The Admiral'. Videos on social media also appeared to show other nearby boats coming to the rescue as people fled the sinking ship for the water.

Local media reports suggested that the boat had been overloaded and that none of the passengers were wearing life vests. Colombia's chief of police said that 30 people were missing after the wreck.

The head of Antioquia state's disaster response agency told AP that nine people had died in the accident and 28 were still missing. She added that 99 people were rescued while 40 managed to get to shore themselves.

A long holiday weekend in Colombia meant that the resort was particularly busy on Sunday.