A lava dome is growing in the volcano's crater, the National Disaster Prevention Center said Tuesday. The 17,886-foot (5,450-meter) volcano also has been spewing fragments of incandescent rock recently, as well as water vapor and ash.
The volcano could experience "significant explosions of growing intensity that hurl incandescent rocks significant distances," large ash showers and possible flows of mud and molten rocks down the volcano's flanks, the center said.
The agency said the area has been closed to visitors and urged people to stay at least seven miles (12 kilometers) from the crater, which is about 40 miles (65 kilometers) southeast of Mexico's capital. It also recommended that people in the surrounding areas clean ash from weak rooftops and to cover their mouths to avoid inhaling it.
The alert is now at the highest level of the yellow stage; the next stage is a red alert, which presumably would prompt evacuations to begin.
Known as "El Popo," the volcano staged its most violent eruption in 1,200 years on Dec. 18, 2000, when an explosion sent up a plume of red-hot rock and forced the evacuation of thousands of people who live at the volcano's base.
The volcano, which seats in the central states of Mexico, Puebla, and Morelos, has been erupting intermittently since December 1994.