Five people are feared dead after one of the Philippines' most active volcanoes spewed a giant ash cloud on Tuesday, with foreign tourists on its slopes during the eruption, authorities said.

National Disaster Risk Reduction and Management Council chief Eduardo del Rosario said local officials reported that up to 20 climbers and guides, including foreigners, were on Mount Mayon when it erupted.

"Five killed and seven are injured, that is the latest report," del Rosario said. "We do not have the identity of those killed or injured, whether they are foreigners or locals."

He said rescuers could not confirm firsthand the reported casualties until they reached the site.

Joey Salceda, governor of eastern Albay province where the 2,460-metre (8,070-foot) Mayon is located, gave the same number of casualties.

Marti Calleja, a local tour operator, said his company had sought help to rescue five foreigners who had gone to climb Mayon with his outfit's guides shortly before the explosion.

"It rained like hell with stones. It was sudden and there was no warning," Calleja told AFP by telephone.

The group was trapped about half a kilometre (a third of a mile) below the crater, Calleja added.

Chief state seismologist Renato Solidum said people living around Mount Mayon did not need to evacuate, describing the event as a "minor steam-driven explosion" of ash deposits at the crater mouth making contact with rain.

Mayon spewed a thick column of ash 500 metres (1,600 feet) into the air, witnesses told AFP.

"There is no magma activity. Essentially what happened today is a normal process of a steam-driven explosion," Solidum told the television station.

Residents in towns around the picturesque volcano famous for its near-perfect cone said they were surprised by the sudden activity, which came as many were having breakfast.

"It was so sudden that many of us panicked," Jun Marana, a 46-year-old bus driver and father of two, told AFP by telephone.

"When we stepped out we saw this huge column against the blue sky."

Marana said the ash column was dispersed by winds after about an hour, but said he was not taking his chances and was prepared to leave his home anytime.

Mayon, about 330 kilometres (200 miles) southeast of Manila, has erupted 48 times in recorded history.

In 1814, more than 1,200 people were killed when lava flows buried the town of Cagsawa.

In December 2009 tens of thousands of villagers were displaced when Mayon spewed ash and lava.

The volcano also erupted in August 2006. There were no direct deaths caused by the explosion, but the following December a passing typhoon unleashed an avalanche of volcanic mud from its slopes that killed 1,000 people.