The Philippines raised the alert for the Mayon volcano early Sunday, citing signs of rising magma.
The raising of the alert level means the current volcanic activity could be of "magmatic origin, which could lead to more phreatic eruptions or eventually to hazardous magmatic eruptions".
Mayon, one of the most active volcanoes in the country, first spewed ashes, which reached 2.5 kilometers (1.5 miles) in a phreatic eruption, lasting for nearly two hours on Saturday afternoon.
"The second phreatic eruption produced a greyish steam and ash plume that was largely obscured by summit clouds", the institute said, adding that a strong smell of sulfur pervaded affected areas.
For the third time in less than 24 hours, a phreatic eruption occurred at Mount Mayon before noon today.
"Since the eruption, rockfall events have been intermittently recorded" while a "faint crater glow" was detected late Saturday night.
Officials said residents should protect against inhaling the ash, advising them to wear masks or cover their noses and mouths with wet clothes.
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Traces of ash fell on Barangay Anoling, Daraga where rumbling sounds were heard; Barangays Sua, Quirangay, Tumpa, Ilawod and Salugan in Camalig, where residents in the town center also reported a sulfuric odor; and Barangays Tandarora, Maninila and Travesia in Guinobatan.
The volcano's most destructive eruption took place in February 1841 when lava covered a nearby town and killed 1,200 people.
With the raising of the alert level, people are advised against entering the 6-kilometer permanent danger zone, and people living in the volcano's southwest and west sides should prepare for possible ash fall events. It last erupted in 2014, spewing lava and forcing thousands of people to evacuate.
Despite this, Cedric Daep, head of the Albay Provincial Disaster Risk Reduction and Management Office, said people in evacuation centres may be allowed to return home later today, unless Phivolcs raised the volcano alert level further.
He went on: "We have not reached the critical level".
Renato Solidum, the head of PHIVOLCS, declared the Mayon is due for another eruption after having displayed abnormal behaviour since the end of past year.