The tourist trade faces strict penalties for allowing visitors onto a volcano island near the Philippine capital after it began to show signs that it might erupt, the coastguard said on Sunday.

Resort and boat operators who violate the ban by bringing tourists to Taal may have their vessels confiscated and their resorts closed, said Captain Frankelino Phaeton.

"They (the tourists) are curious, they want to see what is new with Taal but we can't allow that because of the danger," Phaeton, the local coastguard operations officer told AFP.

Despite earlier warnings from government vulcanologists to avoid the volcano, foreign tourists were still coming to the island, often hiring horses to ride up to the volcano's crater, local coastguard personnel said.

It was mostly South Koreans and some Japanese who were visiting the restive island, they added.

Under the new rules, resort and boat operators can only bring tourists halfway to the island but cannot land, Phaeton said.

The ban was imposed after government volcanologists warned on Tuesday that Taal, 65 kilometres (40 miles) south of Manila in the centre of a lake, was becoming more active and showing signs it could erupt.

Taal 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 five-level alert to level two. 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.

Some residents of the volcanic island have voluntarily left, said Phaeton, but there has been no order yet to forcibly evacuate those remaining.