Ash from Popocatépetl volcano, which is in a central part of the country, has delayed flights in Mexico City and led authorities to close schools in dozens of municipalities.
Some 25 million people live in a 60-mile (96km) radius of the volcano, which is about 72km south-east of Mexico City and located between the states of Morelos, Puebla and state of Mexico.
On Sunday, the National Civil Protection Coordination (CNPC) increased its volcanic threat level to "yellow phase three," which calls on some three million people who live in towns and villages adjacent to the volcano to stay alert and prepare for a possible evacuation.
The CNPC also warned of greater volcanic activity and the dispersion of ash into several nearby towns at high speed. It also recommended that residents nearby avoid outdoor activities.
"In the case of Mexico City, the risk is ash fall. We are prepared for that scenario and we know what to do. Let's stay alert," Mexico City's mayor Claudia Sheinbaum said in a tweet on Sunday.
Mexico City's Benito Juárez International Airport (AICM) was temporarily closed on Saturday and some flights were delayed on Monday due to the presence of volcanic ash, according to airport authorities.
"Air travellers may continue to experience additional flight delays, cancellations, or temporary airport closures with little or no warning," the US embassy in Mexico said, with the airport also issuing a similar alert.
State authorities have also suspended in-person classes in schools across 40 municipalities in Puebla, five municipalities in the state of Mexico and seven in Tlaxcala and said the classes will be held remotely.
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The US embassy in Mexico issued a warning on Saturday saying the Popocatépetl volcano "has exhibited increased activity since May 15, registering hundreds of tremors and smoke and ash exhalations."
The embassy also recommended people to not travel within a 12km radius of the volcano, citing warnings from local authorities.
The volcano had been dormant for decades until it erupted in 1994. Since then, its rumblings have become a part of daily life for residents.
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