The Philippines’ most-active volcano explosion Monday, sending a huge plume of gray ash, steam and lava fragments into the sky and prompting officials to warn that a major eruption may be imminent.

Superheated lava, molten rocks and steam cascaded down Mount Mayon and covered nearby villages in darkness, said Renato Solidum, an official with the Philippine Institute of Seismology and Volcanology.

The explosion forced authorities to raise the alert level to four on a scale of five. The increase means an explosive eruption could be possible within hours or days.

"If the eruption is vertical, it's possible pyroclastic flows or pyroclastic density currents may cascade down in all directions," Soldium said, adding that villagers and tourists should stay away from the volcano.

Soldium said his agency has observed increased seismic activity and “lava fountaining and a summit explosion” from Sunday, according to Reuters.

More than 27,000 villagers have fled since Mayon started acting up more than a week ago.

Mayon is in coconut-growing Albay province, more than 200 miles southeast of Manila. With its near-perfect cone, it 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 who had ventured near the summit despite warnings. Mayon's first recorded eruption was in 1616 and the most destructive, in 1814, killed 1,200 people and buried the town of Cagsawa in volcanic mud.

The Associated Press contributed to this report.