Nevado del Huila volcano erupted late Tuesday and early Wednesday, provoking avalanches and floods that swept away houses and bridges and prompted thousands to evacuate.
"There are 10,000 people in the zone, of which we've already evacuated 3,500," Luz Amanda Pulido, director of the national disaster office, told The Associated Press after flying over the volcano in southwest Colombia.
There were no reports of deaths or injuries.
The volcano's eruption at 10:37 p.m. Tuesday (0337 GMT Wednesday) was its first on record since Colombia was colonized by the Spanish 500 years ago. A second eruption occurred at 2:57 a.m. (0757 GMT) Wednesday.
Experts were not ruling out more eruptions.
"The seismic activity remains light but permanent, and we can't rule out another bigger event in the next hours or days," said Mario Ballesteros, director of the government's Institute for Geology and Mining.
The eruption sent an avalanche of rocks down the volcano's sides and into the Paez and Simbola rivers, causing them to flood.
"The bridges were swept away, the highway used by the indigenous in the zone was destroyed for various kilometers and the problem we have now is the lack of a route to deliver goods and medicines to the population," Police Gen. Orlando Paez said.
President Alvaro Uribe said he was returning early from the Caribbean port city of Cartagena to visit communities affected by the eruption. The air force said it was preparing to send a planeload of humanitarian aid.
The Nevado del Huila, crested with a crown of ice, is Colombia's third-highest peak at 5,634 meters (18,484 feet). Located 270 kilometers (170 miles) southwest of the capital of Bogota, the volcano awoke in March with a series of internal rumblings.
In 1985, the Nevado del Ruiz volcano exploded, setting off a series of mudslides that wiped the village of Armero away, killing around 21,000 people.