Millions of people in Mexico could be evacuated after increased activity from the country's most dangerous volcano, Popocatepetl.
About 25 million people live within a 96km radius of the mountain. It is located 70km south-west of Mexico City.
Flights were suspended from Puebla airport due to volcanic ash falling on its runways, the state's governor said.
“We don't know what's going to happen,” national Civil Defence Co-ordinator Laura Velazquez said on Monday. “We are prepared for any scenario.”
Shelters are being prepared and Mexico's Defence Department said it could deploy 6,500 troops to assist if necessary.
On Monday, ash plume from the 5,425-metre volcano, known as El Popo stretched hundreds of kilometres to the east, the US National Oceanic and Atmospheric Administration reported.
The National Civil Protection Co-ordination on Sunday increased the volcanic level threat to “phase 3”, which “implies greater destruction in lava domes, persistence of fumaroles and ash fall, as well as explosions of increasing intensity”, Puebla's Governor Sergio Salomon tweeted.
Schools in dozens of municipalities were ordered to close because of the increase in volcanic activity.
Authorities in Mexico City urged residents to stay alert, assuring them that the local government is prepared for ash fall.
The US Embassy in Mexico warned civilians not to travel within a 12km radius of the volcano.
El Popo became active in 1994 after a period of dormancy. The mountain's current period of eruption has been continuing since 2005, Nasa said.
The Associated Press contributed to this report