Nearly 15,000 people have fled from villages around the Philippines' most active volcano as lava flowed down its crater on Monday in a gentle eruption that scientists warned could turn explosive.
Lava flowed at least half a kilometre down a gulley from the crater of Mount Mayon on Monday morning and ash clouds appeared mid-slope as lava fragments rolled down, said Renato Solidum, who heads the volcano institute. It was hard to track down the lava flow given the thick clouds shrouding the volcano.
The Philippine Institute of Volcanology and Seismology increased the alert level for Mount Mayon late on Sunday to three on a scale of five, indicating an increased prospect of a hazardous eruption "within weeks or even days." Molten rocks and lava at the crater lit the night sky in a reddish-orange glow despite the thick cloud cover - a spectacular but also terrifying sight that sent thousands of residents into evacuation shelters.
Disaster-response officials said more than 14,700 people have been moved from high-risk areas in three cities and four towns in an ongoing evacuation. People in the danger area have put up huge white crosses in the past in their neighbourhoods, hoping to protect their lives and homes.
"There are some who still resist but if we reach alert level four, we'll really be obligated to resort to forced evacuation," said Cedric Daep, an Albay emergency official. Level four signifies the volcano could erupt violently within days.
Mayon lies in coconut-growing Albay province about 340 kilometres southeast of Manila. Three steam-driven eruptions since Saturday have spewed ash into nearby villages and may have dislodged solidified lava that was plugging up the crater, causing lava to start gushing out, Mr Solidum said.
Lava last flowed out of Mayon in 2014 when 63,000 people fled from their homes.
"We think the lava now is more fluid than in 2014. This means the flow can reach further down [the slopes] at a faster rate," said Mr Solidum said. "We see similarity with eruptions where the first phase of the activity started with lava flow and culminated in an explosive or hazardous part. That's what we are trying to monitor and help people avoid."
With its near-perfect cone, Mayon is popular with climbers and tourists but has erupted about 50 times in the last 500 years, sometimes violently. In 2013, an ash eruption killed five climbers, including three Germans, who had ventured near the summit despite warnings of possible danger.
Experts fear a major eruption could trigger pyroclastic flows — superheated gas and volcanic debris that race down the slopes at high speeds, incinerating or vaporising everything in their path. More extensive explosions of ash could drift toward nearby towns and cities, including Legazpi city, the provincial capital, about 15 kilometres away.
The Sunday bulletin said a hazardous eruption was possible within weeks or even days and aircraft have been advised to keep their distance from the volcano. The glow in the crater signified the growth of a new lava dome so the evacuation zone should be enforced due to the dangers of falling rocks, landslides or a collapse of the dome.
"It is dangerous for families to stay in that radius and inhale ash," said Claudio Yucot, head of the region's office of civil defence." Because of continuous rains in past weeks, debris deposited in the slopes of Mayon could lead to lahar flows. If rain does not stop it could be hazardous." Lahar is the technical term for volcanic mudflows.
Mayon's first recorded eruption was in 1616. The most destructive in 1814 killed 1,200 people and buried the town of Cagsawa in volcanic mud. The belfry of a Cagsawa church juts out of the ground in a reminder of Mayon's deadly fury and has become a tourist attraction.