Two similar phreatic eruptions occurred at the volcano in central Albay province on Sunday, unleashing more ash.
PHIVOLCS raised alert level 2 over Mayon Volcano early Sunday, meaning there is increasing unrest, after the volcano spewed ash on Saturday afternoon.
However, he said, the heavy rains have made it hard for them to collect data.
And just a few hours later, the threat was raised again from two to three as Phivolcs warned of "hazardous eruption" within weeks or days as Mayon Volcano has exhibited "high unrest" and its magma is at the crater. "Faint crater glow has been first observed at 10:16 p.m.", PHIVOLCS said.
PHIVOLCS said traces of ash fell on Barangay Anoling in Daraga, Barangays Sua, Quirangay, Tumpa, Ilawod, Salugan of Camalig, and Barangays Tandarora, Maninila, and Travesia in Guinobatan. It was at its most deadly in February 1841, when 1200 people were killed and lava buried a town.
Authorities advised people to cover their noses and mouths with a damp, clean cloth or dust mask if they were exposed to ash from the eruptions, and said aircraft must avoid flying close to the volcano's summit.
Under Alert Level 3, the extension of danger zone in the sector where the crater rim is low will be considered.
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I became ... irritated and fearful for the young lady. "She was sitting on the floor". I'm like, 'come on, let's go over here. She deserves to get some type of help.
Mayon last erupted in 2014, with lava flows forcing thousands of people to evacuate.
Daraga town mayor Gerry Jaucian also ordered residents and the public to refrain from engaging in any human activity within the volcano's six-kilometer permanent danger zone.
Phivolcs chief Renato Solidum said the volcano appeared due for another eruption as it has been displaying abnormal behavior since late past year.
Earlier, Phivolcs said in a separate advisory that the Mayon Volcano also had a phreatic eruption at 8:49 a.m. that lasted for 5 minutes.
Those within the slope of the volcano, but outside the danger zone, were told to take precautionary measures against possible roof collapses due to accumulated ash and rainwater.
National Disaster Risk Reduction and Management Council (NDRRMC) Spokesperson Romina Marasigan said most of the 919 families evacuated were temporarily sheltered at schools in Camalig and Guinobatan.
But Daep said those in evacuation centers may be allowed to return to their homes later in the day, unless Phivolcs raises the volcano alert level further.