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More than 9,000 people have evacuated the area around the Philippines' most active volcano as lava flows down its crater in a gentle eruption that scientists are warning 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.

Institute head Renato Solidum said 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.

Molten rocks and lava at Mayon's crater lit the night sky Sunday despite thick clouds, sending thousands of residents into evacuation shelters.

Mayon in northeastern Albay province has erupted about 50 times in the last 500 years.