At least six people were dead and as many as 31 others were missing after a boat carrying about 160 people sank in Colombia on Sunday, authorities said.

President Juan Manuel Santos, who visited the scene Sunday night at a reservoir in Antioquia near Medellin, told reporters that six people had died and that 31 others were missing.

Rescuers search for victims after a tourist boat sank with 150 passengers aboard Sunday in the Guatape reservoir in Colombia. Fredy Builes / Reuters

Authorities in Antioquia had earlier said nine people were believed to have died, but near midnight Sunday, Luis Pérez Gutiérrez, the governor of Antioquia, tweeted: "The latest report indicates 6 people killed."

Gutiérrez said there was no official registry of how many people were aboard the boat, the El Almirante, but authorities said they believed it was carrying about 160 people.

Santos said relief agencies were assisting. Specialized divers were searching for the missing.

The reservoir, a short drive east of Medellin, is popular with kayakers, paddle boarders and tour boat operators. This weekend is a long one — Monday is a national holiday — and the area was packed with revelers, The Associated Press reported.

A local restaurant owner, Daniel Giraldo, told the AP that he rushed to shore after hearing ambulance sirens. By the time he got there, the boat was already submerged.

"It sank in a matter of four minutes," he said.

Video posted online showed the colorful, four-level boat listing, then disappearing into the water. A large group of people can be seen on a roof deck as smaller vessels and jet skis race toward them, apparently to help rescue passengers.

It wasn't immediately clear why the boat sank, but a local television network, Teleantioquia, reported the vessel was overcrowded: It was designed for a maximum of 150 passengers, but 170 were aboard. According to the AP, none of them were wearing life vests.

It was unclear if any Americans were affected in the incident, a U.S. State Department official told NBC News.