Authorities said there was no imminent threat of a major eruption of the restive Taal Volcano in Batangas province, south of Manila.

But they said its emission of sulphur dioxide-laden steam in recent days has impacted dozens of students in nearby towns, causing skin, throat and eye irritation.

Randy Dela Paz, operations section chief of the civil defence's southern Manila office, told DWPM radio they received reports of respiratory illnesses due to the volcanic smog.

Classes were suspended in the capital region and in dozens of towns and cities in Cavite, Laguna, and Batangas provinces, with some schools resuming online classes and home learning that were in wide use at the height of the COVID pandemic.

The aviation authority on Friday told pilots to avoid flying close to the volcano's summit.

Images showed Manila and its surrounding regions shrouded in a haze as the Philippine Institute of Volcanology and Seismology said it had observed hot volcanic fluids in the volcano's crater lake, resulting in the emission of volcanic gases.

It said the alert remained at Level 1 on a scale of one to five, denoting a "slight increase in volcanic earthquake, and steam or gas activity".

Taal, one of the world's smallest volcanoes, is among two dozen active volcanoes in the Philippines.

It lies along the Pacific "Ring of Fire", a seismically active region prone to earthquakes and volcanic eruptions.

In January 2020, Taal spewed a column of ash and steam almost 10 miles high, forcing more than 100,000 people to evacuate and dozens of flights to be cancelled as heavy ash fell.