A large ash explosion from Bulusan, a 1 559m volcano, drifted south to cover Irosin, a town of 46 000 people, said Harriet Sayson, of the civil defence operations centre in nearby Legaspi city.
"There's zero visibility at Irosin at the moment, and we can't get in touch with officials there," Sayson said by telephone.
She said civil defence staff were in the town checking for damage or casualties, but none had been reported so far. No evacuations had been ordered, she added.
State volcanologist Ramil Vaquilar said rumbling sounds accompanied an ash column that rose between two and 2.5km above the crater.
"An eruption occurred at 09:12," he said, adding no further major activity had been observed since.
Bulusan is among 23 active volcanoes in the Philippines, which is located in the so-called Ring of Fire of volcanic activity around the Pacific.
Bulusan last erupted between March and June of 2006.