A giant mushroom-shaped cloud shot up from the Philippines' most active volcano on Monday, turning day into night as it rained ash on communities where tens of thousands have fled after warnings of an impending eruption.

"Hazardous eruption imminent," the state volcanology agency concluded in its latest bulletin, saying Mayon volcano could blow up within days after two weeks of activity.

Fine ash and sand fell on Legazpi, a city of about 200,000 people, and nearby areas after the midday explosion turned the area into virtual nighttime, forcing motorists to switch on their lights and use windscreen wipers, an AFP video stringer said.

The ash column rose several kilometres above the volcano, blotting out the sun in a largely agricultural region some 330 kilometres (205 miles) southeast of Manila.

"I had to stop because my helmet had filled up with ash," local housewife Girlie Panesa, 39, told AFP as she parked her motorcycle by the roadside in the nearby town of Ligao.

She asked bystanders for water to wash the cement-grey ash off her visor, saying she plans to ride home despite the hazardous conditions because her teenage daughter was alone in their house.

"We expect the explosions to continue," Philippine Institute of Volcanology and Seismology director Renato Solidum told a news conference in Manila.

"There is a possibility of a dangerous eruption, the start of which we are already witnessing," Solidum added.

He advised local officials to evacuate more areas around the crater, expanding the danger zone from six kilometres to eight kilometres.

He also warned aircraft to steer clear of the area due to the danger of jet engines sucking in ash which could gum up turbines, potentially causing a catastrophic crash.

The Civil Aviation Authority of the Philippines (CAAP) said it shut down Legazpi airport until further notice, disrupting several domestic flights, while warning off other aircraft from the region.

"CAAP advised pilots flying near the area to exercise extreme caution, as ash from volcanic eruption can be hazardous to the aircraft," it said in an advisory.

More than 40,000 people had fled in the past week, the civil defence office in Manila said Monday.

Solidum said superheated volcanic rocks and ash rolled down the volcano's flanks while the ash column was shooting up, threatening surrounding communities.

Mayon, a near-perfect cone, rises 2,460 metres (8,070 feet) and is considered the most volatile of the country's 22 active volcanoes.

There have been 51 previous eruptions in recorded history, the last one in 2014. In 1814 it buried the town of Cagsawa, killing more than 1,000 people.

The Philippines is part of the Pacific "Ring of Fire" of islands that were formed by volcanic activity.

The most powerful explosion in recent years was the 1991 eruption of Mount Pinatubo, about 100 kilometres northwest of Manila, which killed more than 800 people.

Source: AFP