The stoplight warning system for the Popocatépetl volcano in Mexico rose to yellow phase 3 on Sunday, according to officials.
In a tweet, the government shared a chart showing that alarm was just below the “red” level.
The phase includes large domes developing and exploding with increasing intensity, sending rock into the air and pyroclastic material down the sides.
For most of the years since the stoplight was introduced, it has been stuck at some stage of “yellow.”
On Tuesday morning, the government tweeted that there is no visibility toward the stratovolcano, which is situated about 40 miles southeast of Mexico City, but that it “has been possible to observe an emission with a low ash content.”
“The tremor episode of variable amplitude recorded during the last hours continues,” it said.
Officials warned at a press conference the day before that there was “no risk to the population at this time.”
On Monday afternoon, the Hermanos Serdán International Airport was reported to have suspended operations due to the presence of ash falling on its runways. This comes after Mexico City’s two main airports temporarily shut down on Saturday due to the ash.
In addition, alert protocols were activated in hospital and medical units due to the alert phase.
The Federal Electricity Commission said it was ready to deal with possible effects on the electricity supply in the states of Puebla and Tlaxcala due to the volcanic activity.
Residents were urged not to go close to the volcano, where there was an occasional low rumble on Monday.
Ash has descended and reduced visibility and the 17,797-foot-tall mountain has become increasingly explosive for more than a week. The plume has extended over hundreds of miles to the east and over the Bay of Campeche, according to the U.S. National Oceanic and Atmospheric Administration.
Schools were closed in dozens of municipalities across three states and evacuation drills were held on Monday.
The Defense Department said it was ready to activate 6,500 troops and shelters were being prepared.
“We are standing by,” President Andrés Manuel López Obrador assured Monday.
Popocatépetl came to life in 1994 and has experienced periods of greater activity from 2000 to 2003 and 2012 to 2016.
The Associated Press contributed to this report.