BOGOTA—Colombian authorities have raised the alert level for the deadly Nevado del Ruiz volcano to orange, suggesting a greater probability of an eruption in the coming days or weeks, due to an increase in seismic activity.

A significant increase in seismic activity associated with rock fracturing within the volcanic edifice has been registered since March 24, the Colombian Geological Service (SGC) said in a statement late on Thursday.

The Nevado del Ruiz is a stratovolcano, or composite volcano, which straddles the border between Colombia’s Tolima and Caldas provinces.

“We have an orange alert due to the change in seismology at the Ruiz volcano. Mayors must prepare the protocols for this state of alert,” President Gustavo Petro said in a Twitter message on Friday.

Famous examples of catastrophic stratovolcano eruptions include Krakatoa in Indonesia, which exploded in 1883, and Vesuvius, in Italy, which destroyed the Roman cities of Pompeii and Herculaneum in 79 AD.

Colombia’s biggest ever natural disaster occurred in 1985, when Nevado del Ruiz erupted, melting ice and snow which resulted in flows of water and rock fragments, killing more than 25,000 people.

According to the Global Volcanism Program of the U.S. Smithsonian Institution, the eruption in 1985 resulted in the fourth-largest single-eruption death toll ever recorded.