About 120 residents living near Colombia's Nevada del Ruiz volcano were being evacuated Monday, officials said, after increased seismic activity.

A 1985 eruption by the volcano that killed 25,000 people is considered the worst natural disaster in Colombian history and one of the deadliest eruptions of the 20th century.

The governor of the Caldas department, Luis Carlos Velasquez, told Blu Radio that around 40 families living near the volcano would be relocated as a "preventative" measure.

Some of the homes near the smoking crater are not reachable via cell phone or radio, he said, making it difficult to warn them in case of an emergency.

There has recently been a "significant increase in seismic activity," the ministry of mines said in a statement last week while Colombia's SGC geological service warned of a "probable eruption within days or weeks."

And President Gustavo Petro tweeted: "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."

According to Colombia's UNGRD disaster risk management unit, the orange level meant the likelihood of an eruption "bigger than any in the last decade."