Mexico City, May 22 (IANS): Mexican authorities have raised the alert level due to the recent increased activity of the Popocatepetl volcano, one of the country's most active.

The alert level was raised from yellow phase two to yellow phase three, one step below the red alert, Laura Velazquez, head of the Mexican National Civil Protection Coordination Agency, said at a press conference on Sunday.

The yellow phase three alert signals "intermediate to high" volcanic activity such as persistent plumes of smoke, gas and light ashfall, along with explosions of increasing intensity with the launch of incandescent rocks.

Popocatepetl, located about 70 km southeast of Mexico City, has registered more frequent exhalations and even moderate explosions accompanied by the expulsion of ash and incandescent rocks.

Ashfall from exhalations and explosions forced the closure of Mexico City's two main airports for several hours on on May 20.

The volcano, standing over 5,400 metres above sea level, straddles Puebla, Morelos and the State of Mexico.

The towering stratovolcano has been erupting since January 2005, with near constant venting from fumaroles, punctuated by minor steam, gas, and ash emissions.