A major eruption of a volatile Philippine volcano could be just weeks away, scientists warned Thursday, as thousands of residents faced the prospect of four months in temporary shelters.

"We saw the formation of two lava domes at the crater of Mayon during an aerial survey yesterday," government volcanologist July Sabit told AFP.

"This indicates pressure is building up from the inside and the rising magma had accumulated at the mouth to form the domes," he said.

He said it was likely that the domes would collapse if the pressure continued to build, causing a major eruption or fast-moving avalanches of hot volcanic ash, rock and gases, known as pyroclastic flows.

This could also mean that tens of thousands of villagers evacuated from around the picturesque but deadly Mayon could spend up to four months crammed in temporary shelters, officials said.

Authorities had earlier announced level three of a five-step alert system on Mayon, meaning a hazardous eruption was likely after the 2,460-metre (8,070-feet) volcano famed for its near-perfect cone began belching ash, steam and lava this week.

Sabit said lava slowly cascading down Mayon's south-eastern flank was coming from the domes.

However, he said scientists have not seen "intensified lava fountainings, or big earthquakes" that could precipitate a sudden huge explosion and a raising of the alert level.

Chief government volcanologist Renato Solidum said lava has cascaded about 1.5 kilometres from the summit, punctuated by small ash explosions over the past 24 hours.

He said the current pattern was similar to Mayon's last eruption in 2006, when it expelled lava and steam for two months lasting until the first week of September.

No one died directly from that explosion, but a powerful typhoon three months later dislodged tons of debris from the slopes, unleashing devastating mudslides that left over 1,000 dead.

"We will fully evaluate the volcano as the non-explosive phase progresses," Solidum said.

Government seismic monitors meanwhile said 82 small earthquakes were recorded overnight but the deep rumblings were mostly generated by the movement of lava and incandescent debris that lit up the peak.

This was preceded by five minor explosions on Wednesday, one of them sending a plume of ash 500 metres (1,600 feet) into the air.

As a precautionary measure, the military and police have been evacuating an estimated 50,000 residents, mostly from farming villages in Mayon's fertile foothills.

By late Wednesday, an estimated 30,760 people had been relocated to public schools converted into evacuation areas beyond an eight-kilometre (five miles) radius danger zone.

"We are continuing our evacuations today," Raffy Alejandro, the chief civil defence official in the region told AFP.

"We aim to be able to evacuate everyone out of harm's way by the end of today, or tomorrow.

"Our best protection against any pyroclastic flow is distance and all our evacuation sites are beyond the danger zone," Alejandro said.

He said the provincial government of Albay, 330 kilometres (220 miles) southeast of Manila, where Mayon is located, has resources enough to last only four weeks.

But with an explosion likely, residents may be displaced for up to four months, and help will be needed from the national government and international humanitarian agencies, Alejandro said.

"We can't afford to bring them back with Mayon's unpredictable behaviour," he said. "They will have to spend Christmas and New Year in evacuation centres."

Mayon's name is derived from the word 'magayon' which in Albay dialect means beautiful. It has erupted 48 times since records began, claiming thousands of lives. In 1814, more than 1,200 people were killed as lava buried the town of Cagsawa.

The Philippines, which has 22 active volcanoes, lies on the Pacific "Ring of Fire" that is known for its volcanic activity.