The Philippines raised the alert on its most active volcano to the second-highest level on Monday and ordered the evacuation of around 20,000 people after three ash explosions signaled a possible eruption within days.

Mayon volcano has been spewing lava and boulders the size of cars since last month, leaving a bubbling, pyrotechnic trail more than 6 km down its southeastern slope.

"We deem it necessary to raise the alert level to four because there were three ash explosions this morning," Renato Solidum, director of the Philippine Institute of Volcanology and Seismology (PHIVOLCS), told Reuters.

"That would mean that the hazardous eruption is more likely."

The military sent trucks to ferry thousands of villagers to evacuation centers outside an eight-kilometer danger zone on the southeast of the 2,462-meter-high volcano.

A 6 km no-go area, fringed by coconut plantations, encircles the mountain.

Residents reported seeing Mayon, famed for its near-perfect cone shape, belching cauliflower-like plumes of ash and gases 800 meters high this morning.

Disaster officials have previously said that around 60,000 people in Albay province would be evacuated in the event of a major eruption with lava moving as fast as 60 km/h.

The Philippines lies on the "Ring of Fire", a belt of volcanoes circling the Pacific Ocean that is also prone to earthquakes.

Mayon is the most active of 22 volcanoes in the country, having erupted around 50 times over the past four centuries. The most destructive eruption came in February 1841 when lava flows buried a town and killed 1,200 people.

The last time Mayon erupted was in 2000-2001. Reuters