Manila, Jun 12 (IANS): A volcano in the Philippines' Sorsogon province spewed ash on Sunday, a week after a phreatic eruption that prompted authorities to raise the alert level and evacuate residents to safety.

Phreatic eruption is a steam-driven explosion that occurs when water beneath the ground or surface is directly heated by hot rocks or new volcanic deposits or indirectly by magma or magmatic gas.

The Philippine Institute of Volcanology and Seismology said it detected the phreatic explosion in the Bulusan volcano at 3.37 a.m. on Sunday, reports Xinhua news agency.

"The event lasted 18 minutes based on the seismic record, but the eruption plume was not visible in camera monitors," the institute said.

On June 5, authorities evacuated more than 200 residents near the volcano after spilling a grey plume about a kilometre high into the sky.

The phreatic eruption lasted approximately 17 minutes and ashfall was reported in towns near the volcano.

The volcano remains on alert level 1 on a scale of 5, meaning that the volcano is still in an "abnormal condition" .

The Institute urged people to stay away from the 4 km "permanent danger zone".

It also advised pilots to avoid flying close to the volcano's summit as ash from any sudden phreatic eruption can be hazardous to aircraft.

Bulusan, located approximately 390 km southeast of Manila, has erupted 15 times since 1885 and is the fourth most active volcano in the Philippines after Mayon, Taal, and Kanlaon.