People living on its slopes told of hearing "strong, strange noises" coming from the summit of the 18,500-foot-high volcano Friday and Saturday, the BBC reported.
Hot lava from the volcano's last eruption in 1985 melted glaciers and snow and caused mudslides that killed more than 25,000 people.
Scientists at the Volcanic Observatory in the nearby town of Manizales said the plume of ash and gas had risen to a height of 6,500 feet on Sunday and said there was a risk of an eruption "within days or weeks."
About 150 families considered most at risk have been evacuated from along rivers that flow from the flanks of the volcano, officials said.
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