Jaime Sincioco of the Philippine Institute of Volcanology and Seismology said on Wednesday that the Bulusan volcano in Sorsogon province ejected ash on Tuesday night.
"Last night, at 10.58pm, there was a phreatic or steam-driven explosion, producing ash clouds 1.5km from the summit. That's just a mild ash emission," Sincioco said. Mild volcanic quakes were also felt, he said, and steam was seen coming from the crater early on Wednesday.
He warned residents not to venture within the 4km danger zone due to fears of sudden explosions.
"We are still monitoring the volcano. We will raise the alert level as appropriate," Sincioco said. "The present manifestation still does not indicate that a major activity is imminent."
The institute issued a level-1 alert for Bulusan on Sunday after their instruments detected successive quakes.
The highest level of alert is 5, indicating lava flows or ash columns reaching 6km.
"We are still monitoring the volcano. We will raise the alert level as appropriate"
Jaime Sincioco of the Philippine Institute of Volcanology and Seismology
Bulusan, which made similar explosions from November 1994 to January 1995, is one of six of the most active volcanoes monitored by government scientists.
The Philippines lies in an area in the Pacific basin that is vulnerable to earthquakes and volcanic eruptions.
Mount Pinatubo, on Luzon island in the northern Philippines, erupted in 1991 in the century's biggest blast, burying dozens of villages under tonnes of mud after lying dormant for 600 years.
More than 800 people died in the wake of Pinatubo's eruption, mostly from diseases in overcrowded evacuation camps.
News updates are available at the Philippine Institute of Volcanology and Seismology