Colombia on Sunday issued a warning for areas crossed by rivers that pass through the Nevado del Ruiz volcano area, amid heavy rains and concern that an eruption could be in the works.

Authorities issued a "red alert" for rivers near the volcano in Caldas and Tolima departments "due to the change in the eruption threat and to heavy rains in the area," the national weather and environmental institute (IDEAM) said.

The volcano has been rumbling at a greater rate in recent days; its alert level was boosted on March 31.

Saturday, the volcano's activity was still unstable and it was emitting more gases, the National Geological Service said.

The vocano has a tragic past.

On November 13, 1985, the Nevado del Ruiz erupted and melter the Arenas crater; that caused the Guali River to swell and eventually overflow, sweeping over much of the town of Armero in Tolima, killing almost 20,000 of its 29,000 people.

The avalanche at the time killed another 3,000 people in nearby towns.