A volcano close to the Philippine capital is becoming active and may eventually erupt, scientists said Tuesday as they warned tourists to avoid its famous crater.

Taal, located 65 kilometres (40 miles) south of Manila, is one of the most unstable of the country's 22 known active volcanoes with 33 recorded eruptions, the last one in 1977.

Experts raised the second of a five-level alert on Taal on Tuesday. The Philippine Institute of Volcanology and Seismology said volcanic quakes in the area have been occurring since April and the crater lake was heating up.

"The recorded high frequency volcanic earthquakes could be the result of active rock fracturing associated with magma intrusion beneath the volcano," it said in a statement.

"Taal volcano's status is now raised to alert level 2, meaning that the volcano is undergoing magmatic intrusion which could eventually lead to an eruption."

Taal's crater is surrounded by a lake about 15 kilometres off the hill resort of Tagaytay, a major tourist destination.

"Phivolcs reminds the general public that the main crater remains off-limits because hazardous steam-driven explosions may occur, along with the possible build-up of toxic gases," the institute said.

It said a trail leading to the crater was "hazardous".

However the lake surrounding the crater has prevented deaths in the 1977 and other eruptions, as the body of water has protected outlying areas from the lava.