The lava flow, which is composed of volcanic debris and fast-moving hot gases, comes after the Philippine Institute of Volcanology and Seismology (Philvolcs) late Sunday raised the alert level on Mayon once again -- from level 2 to 3 -- which means a "hazardous eruption is possible within weeks or even days".

Level 5 represents the highest warning, which means an eruption is ongoing.

The raise of the alert level came after the volcano’s "relatively high unrest" with three recorded phreatic eruptions and 158 rockfall events since Saturday afternoon.

"Technically, Mayon has been erupting non-explosive magmatic eruption with lava flow being issued out of the summit crater," Philvolcs director Renato Solidum said as quoted by ABS-CBN News.

The latest volcanic activity has brought ash fall in the towns located on the southwestern flank of Mayon’s crater.

Some 12,000 people have fled to evacuation centers located on safer grounds, the National Disaster Risk Reduction and Management Council reported Monday.

Authorities have also extended the danger zone from 6 kilometers (3.7 miles) around Mayon to 8 kilometers (5 miles) following the lava flow.

The local disaster agency has ordered the suspension of classes in the entire province of Albay due to the possible major eruption of the volcano as well as heavy rains.

Mayon, renowned for its perfect cone, last erupted in 2014. It also erupted in 2013, killing four European mountain climbers and a Filipino guide.

Its most destructive eruption was in February 1841, when lava flows buried a town and killed 1,200 people. Minor eruptions were recorded in 1984, 1993, 2008, 2009 and 2010.