"It is a quiet eruption as of now," said Renato Solidum, chief of the Philippine Institute of Volcanology and Seismology, who ordered the alert level raised after observers saw "lava trickles" flowing down the slopes of the 8,118-foot volcano.
"A hazardous eruption is possible. We don't know when, maybe within weeks," Solidum said.
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He said scientists had expected lava flows after ash shot out of the volcano Thursday, and earthquakes were recorded for two days.
The government maintains a 3 3/4-mile "permanent danger zone" around the volcano's crater, but many residents still live or farm on its slopes. There was no immediate word on evacuations.
Mayon, one of the country's 22 active volcanos, last came to life in a series of eruptions in 2001, forcing the evacuation of about 50,000 people. It has erupted about 50 times since 1616.
A major tourist attraction for its near perfect cone, the volcano is about 210 miles southeast of Manila.