A Philippines volcano that killed five people last month in a huge ash explosion is showing renewed signs of activity, a government volcanologist warned Saturday.

The picturesque Mayon Volcano is bulging near its edifice, emitting more gas, and has a noticeable glow at its cone, resident volcanologist Ed Laguerta said.

"Activity has slightly increased and if there is a further escalation, it would mean raising the alert level around the volcano," he said.

Mayon, one of the Philippines' most active volcanoes, spewed a giant ash cloud and hail of rocks on 7 May, killing four foreign climbers and their Filipino guide.

While the volcano, located about 330 kilometres (200 miles) southeast of Manila, does not show signs of an imminent eruption, it could give rise to another ash explosion or even volcanic quakes that could endanger climbers.

"Even if there is no explosion, there may be rockfalls, or rockslides from quakes," Laguerta said.

Since the deaths last month, the local government has imposed climbing restrictions for the vicinity, but these rules are difficult to enforce said Laguerta.

The 2,460-metre (8,070-foot) Mayon is famed for its near-perfect cone but has a long history of deadly eruptions.

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

The volcano 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.