Three German tourists and their Filipino tour guide were crushed to death when one of the Philippines' most active volcanoes spewed a giant ash cloud and a hail of rocks on Tuesday, authorities said.

Twenty-seven people, including at least nine foreigners, were climbing picturesque Mount Mayon when it erupted without warning, and bad weather meant some of them would have to spend the night on its slopes, officials said.

"It rained like hell with stones," local tour operator Marti Calleja quoted an Austrian woman who was among the injured in the ordeal as saying.

"The rocks that came crashing down on them were as big as dining (table) sets," he told AFP by phone.

Calleja said five foreigners and three of his Filipino guides had begun hiking up Mayon just a few hours before the eruption, which sent a thick column of ash 500 metres (1,600 feet) into the air.

Three Germans aged in their 30s, two men and one woman, and one of the guides from his group were killed, he said.

A 22-year-old Spanish woman also sustained "life-threatening" injuries, while the Austrian woman suffered minor bruises, according to Calleja.

Regional police spokesman Superintendent Renato Bataller confirmed the four fatalities, with seven others injured, including four Thais.

Provincial governor Joey Salceda told AFP that while the volcano had calmed down, rescue helicopters were unable to land due to heavy rain and only four survivors had been taken off the mountain.

He said rescuers were forced to start climbing the mountain to reach the injured, and it was unclear when they would be brought down.

Calleja said the foreigners paid about $100 each for an overnight adventure on the 2,460-metre (8,070-foot) Mayon, which is famed for its near-perfect cone but has a long history of deadly eruptions.

A six-kilometre (3 and 3/4-mile) radius "permanent danger zone" is supposed to be enforced around the volcano. But Calleja said the local government allowed people to climb when there were no signs of an eruption.

"Between 300 and 1,000 climbers go here during the peak season from May to August," Calleja said.

Salceda confirmed the arrangement, but said tourists hiking up should still inform the authorities beforehand.

"Mayon is just like any other mountain, and mountaineers assume the same risk as anywhere. But while we allow them to go, they should notify us and seek our approval. In this case, they did not," he said.

Volcanologists described the eruption as a 73-second "steam-driven minor explosion" that was not expected to be repeated anytime soon.

Chief state seismologist Renato Solidum said people living around Mayon did not need to evacuate. He said the explosion was triggered when rainwater made contact with hot ash deposits on the crater mouth.

Residents in towns around the volcano said they were taken by surprise.

"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."

Mayon, about 330 kilometres (200 miles) southeast of Manila, has erupted dozens of 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.