Philippine scientists said that a "hazardous eruption" of a volcano in the archipelago could be days or weeks away, and urged the evacuation of nearby residents from their homes.
Hundreds of families living around Mount Mayon in central Albay province are expected to be moved to safer areas after the Philippine Institute of Volcanology and Seismology raised the alarm.
Mayon, a near-perfect cone located about 330 kilometres (205 miles) southeast of the capital Manila, is considered one of the most volatile of the country's 24 active volcanoes.
The seismology agency said it observed three fast-moving avalanches of volcanic ash, rock and gases, known as pyroclastic density currents (PDCs), on Mayon's slopes on Thursday.
There are "increased chances of lava flows and hazardous PDCs... and of potential explosive activity within weeks or even days", the agency said, raising the alert level from two to three on a scale of zero to five.
"All necessary preparations are being done," said Eugene Escobar, the Albay provincial disaster management agency's officer-in-charge.
Rommel Negrete, an officer for the agency, said residents would be evacuated from Anoling village on the volcano's slopes.
Meanwhile, Taal volcano, located about 50 kilometres (31 miles) south of Manila, has been releasing sulphur dioxide this week, blanketing surrounding areas in smog and prompting warnings for people to stay indoors.
Steam-rich plumes have been recorded rising two kilometres (1.2 miles) into the sky, the seismology agency said on Thursday. It has left the alert level at one.
Earthquakes and volcanic activity are not uncommon in the Philippines due to its position on the Pacific "Ring of Fire", where tectonic plates collide deep below the Earth's surface.
Five years ago, Mayon displaced tens of thousands of people after spewing millions of tonnes of ash, rocks and lava.
The most powerful explosion in recent decades was the 1991 eruption of Mount Pinatubo, about 100 kilometres (62 miles) northwest of Manila, which killed more than 800 people.
It sent out an ash cloud that travelled thousands of kilometres in a matter of days and was blamed for damaging nearly two dozen aircraft.
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© Agence France-Presse
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