By Roy Ramos

ZAMBOANGA, the Philippines

A steam explosion at one of the most active volcanoes in the Philippines may be a precursor to an eruption, officials warned Tuesday.

The Philippine Institute of Volcanology and Seismology raised its alert levels after the explosion occurred at Mt. Kanlaon in the central Philippines.

"Alert level 1 status currently prevails over Kanlaon Volcano, which means that it is at an abnormal condition and has entered a period of unrest," the institute said.

"Local government units and the public are strictly reminded that entry into the 4-kilometer radius permanent danger zone is strictly prohibited due to the possibility of sudden and hazardous steam-driven or phreatic eruptions."

The institute reported that the explosion had occurred in a crater at the summit for eight minutes at 9.55 p.m. (1355 GMT) Monday, based on visual observation.

It added that the volcano had produced a 1-1.5 kilometer (0.6-0.9 mile) high white plume of smoke and a clear rumbling sound.

Four volcanic earthquakes at Kanlaon have been recorded in the past week alone, and after Monday's explosion, volcanic tremors were recorded for five hours.

Civil aviation authorities advised pilots to avoid flying close to the volcano’s summit, as a sudden eruption might endanger aircraft.

Kanlaon is the third most active volcano in the Philippines, having erupted 26 times since record-keeping began -- including an eruption in 1996 that killed three mountain climbers.

The institute has been monitoring three other volcanoes in the Philippines north it says are showing restive signs.