A volcanic eruption in the Philippines continues to spew lava half a mile into the sky, creating lightning bolts, and threatening to erupt again even as tens of thousands of people continue to flee the region and hundreds of flights have been canceled or delayed.

Lava fountains spurting from the Taal Volcano generated tall dark gray, steam-laden plumes that drifted to the general southwest, and experts are predicting continued eruptive activity, the Philippine Institute of Volcanology and Seismology said in a bulletin.

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The alert level since the eruption began Sunday has been a 4, indicating a hazardous eruption is possible in hours to days. (Level 5, the highest, means such an eruption is underway.)

The Philippine Seismic Network recorded a total of 49 volcanic earthquakes in the Taal region in an 8-hour time frame beginning in the early hours of Tuesday morning, and the Institute warned that intense seismic activity coupled with fissuring beneath the Earth's surface could also trigger another eruption.

Overall, there have been 350 volcanic earthquakes recorded near Taal since Sunday, U.N. spokesman Stephane Dujarric said, according to reports by The Associated Press.

Areas around Taal Volcano are advised to guard against the effects of heavy and prolonged ashfall, which has blanketed the region in thick gray matter. More than 38,000 people have been relocated so far to more than 200 evacuation centers.

In addition, more than 500 international and domestic flights were canceled or delayed due to the overnight airport closure, affecting about 80,000 passengers, airport manager Ed Monreal told The Associated Press.

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The Insititute warned aircraft to avoid the airspace around the Taal Volcano, as airborne ash and ballistic fragments from the eruption columns pose hazards to planes in the air.

The Associated Press contributed to this report.