At least five climbers have been killed by rocks and ashes spewed from the eruption of one of the Philippines' most active volcanoes.

Mount Mayon, near the city of Legazpi in central Philippines, erupted as two groups of mountaineers were heading for its crater.

"Five killed and seven are injured, that is the latest report," National Disaster Risk Reduction and Management Council chief Eduardo del Rosario said.

Nearly 30 people, divided into two parties of tourists and Filipino guides, were reportedly shocked by the sudden explosion of rocks, ash and plumes of smokes that jolted the 2,460-metre (8,070-foot) mountain.

"The injured are all foreigners ... They cannot walk. If you can imagine, the boulders there are as big as cars. Some of them slid and rolled down. We will rappel the rescue team, and we will rappel them up again," said Albay provincial Gov. Joey Salceda.

Salceda said three Germans and a Filipino guide were among the dead. Rescuers were trying to bring down the mountain the injured and those who were trapped on the volcano's slopes.

One foreigner was still unaccounted for and presumed dead, Salceda said. One Austrian and two Spaniards were among the rescued and reported small bruises.

The climbers had spent the night camping on the mountain before setting out at daybreak for the crater, and were reportedly hiking about a third of a mile (0.5km) below their target when the volcano rumbled back to life early in the morning.

"It was so sudden that many of us panicked," local resident Jun Marana told AFP. "When we stepped out we saw this huge column against the blue sky."

Guide Kenneth Jesalva told ABS-CBN TV network that rocks "as big as a living room" came raining down, killing and injuring members of his group. Jesalva said he rushed back to the base camp and called for help.

After the eruption, clouds cleared over the volcano and its activity calmed down. No evacuation has been ordered.

Mayon has erupted more than 40 times in the last 400 years. In 1814 more than 1,200 people were killed in the most violent incident on record.

In 2010 the last discharge caused thousands of residents to be moved to temporary shelters as the volcano spewed as far as 8km (5 miles) from the crater. However the trail leading to the near-perfectly conical crater remains a favourite spot for volcano watchers.