Residents of towns in the eastern Philippines have been warned about possible further sudden volcanic eruptions after a volcano spewed a huge, dark cloud of ash which prompted evacuations.

The blast from the Bulusan volcano in the rural Sorsogon province lasted about 17 minutes, sending a grey plume shooting up at least 1km, according to the Philippine Institute of Volcanology and Seismology (PhiVolcs).

No casualties were reported, but authorities raised the alert level to one on the five-level system, indicating "low-level unrest".

A group of 14 hikers and four local guides were midway down the 1,565-metre tall mountain, unaware that an ash cloud was shooting up on the other side, civil defence official Leo Ferreras of nearby Barcelona town told AFP by phone, but all made it down unharmed.

"There was a phreatic eruption of the Bulusan volcano, meaning the explosion was caused by the boiling water under the crater," PhiVolcs head Renato Solidum told local radio DZBB.

Provincial Disaster Risk Reduction Management officers guide vehicles passing through the highway of Juban town in the Sorsogon Province

The local government of Sorsogon, about 500km south of the capital Manila, said ten villages in two towns were affected by the ashfall.

"The evacuation is ongoing there, but our priority is senior citizens and those with asthma," Juban disaster official Dennis Despabiladeras said.

Images of the aftermath showed houses, roads, and trees in Juban town covered in ash, with vehicles struggling to navigate the road due to poor visibility.

Authorities deployed a fire truck to clear the area and residents helped sweep the ash off the roads.

Residents use water to clear ash from their houses

The Manila Airport Authority said no flights have been affected by the eruption so far, though pilots were warned about coming near the area.

Authorities reminded residents that entry into the 4km radius around the volcano is prohibited and advised those living next to it to be cautious "due to the increased possibilities of sudden and hazardous phreatic eruptions".

Residents near the valleys and rivers were also alerted about the danger of mud and stream flows in the event of heavy rainfall.