An explosion has taken place at the most active volcano in the Philippines, sending a huge plume of ash and steam into the sky. Authorities have raised the alert level to four out of five, meaning a potentially deadly major eruption is possible within days.

Mayon, which at 2,462m (8,077ft) is also one of the tallest mountains in the country, has been issuing lava and pyroclastic material since it started showing activity on 13 January.

Almost 40,000 nearby residents have been evacuated in the central province of Albay, and Philippine authorities have closed schools and urged people to stay indoors where possible.

The Philippine Institute of Volcanology and Seismology (Phivolcs) raised its alert on Mayon to level 4, signifying a hazardous eruption is imminent, from level 3, which warns that such an eruption could be "weeks or even days" away.

"We strongly advise all people, both residents and tourists, to avoid the danger zone, and airlines to avoid flying near the volcano summit," agency chief Renato Solidum told a news conference.

The danger zone around the volcano has been expanded to a radius of 8 km (5 miles), he added.

Solidum said the agency had recorded increased seismic activity and "lava fountaining and a summit explosion" from Sunday night, indicating more explosions ahead, including a hazardous eruption.

A level 5 alert signifies a hazardous eruption is underway.

Albay province has run out of emergency funds and more people would be evacuated once government financial help arrived, said provincial governor Al Francis Bichara.

He ordered schools to suspend classes, amid ash fall warnings following the explosion at the volcano.

"In some areas...it's already zero visibility, especially along the foot of the volcano," he told CNN Philippines, adding that strong winds could carry ash to distant towns.

"(People) have to stay home and if they intend to get out of their houses, they have to wear masks," Bichara said.