Mexico City, Mexico — Mexican authorities have started preparing public shelters after seismic activity increased at the Popocatepetl volcano near Mexico City. Authorities have also raised the alert status of towns in the state of Puebla, the state of Mexico, and the capital city.

Mexico's National Disaster Prevention Center (Cenapred) says the white-capped volcano "registered three hours of harmonic tremor at high amplitude, as well as 20 emissions of low to moderate intensity, one of which had an explosive component - spewing a plume of ash and steam more than a half mile into the sky. At times during the evenings, moderate to intense incandescent rock could be seen over the edge of the crater."

According to Cenapred, "his change of activity in the 17,900-foot volcano could lead to the following likely scenarios: intermediate to high scale explosive activity, dome growth and possible lava emission; explosions of growing intensity, occurrence of pyroclastic flows, and notorious ash fall on the closer villages and in lesser amounts in the more remote places, depending on the wind direction."

During the weekend, the government deployed soldiers and federal police to the area - closing off a seven square-mile zone around the foot of the volcano in case of bigger eruption.

The volcano is hidden under cloud and its status has been put at Yellow Phase III, the third highest warning on the center's seven-stage scale.

Popocatepetl has put out small ash eruptions almost every day since 1994 when its current phase of activity began. The eruptions started strengthening two weeks ago and became increasingly more intense over this past weekend.

Authorities in Mexico took similar precautionary measures in April 2012 when seismic activity increased at the volcano.