• The Philippine Institute of Volcanology and Seismology increased the alert level for Mount Mayon.
  • Lava has flowed at least half a kilometre down a gulley from the crater ash clouds appeared mid-slope.

More than 9,000 people have evacuated the area around the Philippines' most active volcano as lava flowed down its crater Monday in a gentle eruption that scientists warned could turn explosive.

The Philippine Institute of Volcanology and Seismology increased the alert level for Mount Mayon late Sunday to three on a scale of five, indicating an increased tendency toward a hazardous eruption.

Lava flowed at least half a kilometer (less than half a mile) down a gulley from the crater and on Monday morning, ash clouds appeared mid-slope, said Renato Solidum, head of the volcano institute.

Molten rocks and lava at Mayon's crater lit the night sky Sunday in an reddish-orange glow despite a shroud of thick clouds that covered the volcano, leaving spectators awed but sending thousands of residents into evacuation shelters.

Albay province emergency response official Cedric Daep said at least 9,000 people have been moved from high-risk areas in an ongoing evacuation. People in the danger area have put up huge white crosses in their neighborhoods, hoping to protect their lives and homes.

"It's risky if people will be left behind," Solidum said of villagers who stay to watch over their homes within a permanent danger zone around the volcano.

But if the institute raises the alert level further, indicating deadlier danger, villagers would have to leave areas near Mayon. The volcano lies in coconut-growing Albay province about 340 kilometers (210 miles) southeast of Manila.

Mount Mayon Philippines volcano eruption
Mount Mayon Philippines volcano eruption
Mount Mayon Philippines volcano eruption

Three steam-explosions since Saturday have spewed ash into nearby villages and may have breached solidified lava at the crater and caused lava to start flowing, Solidum said.

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.

On May 7, 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 vaporizing everything in their path. More extensive explosions of ash could drift toward nearby towns and cities, including Legazpi city, the provincial capital, about 9 miles (15 kilometers) away.

The bulletin sent Sunday night said a hazardous eruption was possible within weeks or even days. It said the glow in the crater signified the growth of a new lava dome and that the evacuation zone be enforced due to the dangers of falling rocks, landslides or a collapse of the dome.

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 that has become a tourist attraction.