A gas reservoir in a burning oil storage facility exploded Thursday, injuring 100 people and touching off a burst of fighting between Christian and Moslem forces that killed three civilians, authorities said.

The deafening blast, heard throughout Beirut, came during the 18th day of fighting between Christian-led army troops and Syrian-backed militiamen who traded mortar and rocket fire.Police said many Beirut residents were taking advantage of a brief lull in the fighting to venture out of their shelters for supplies when the gas explosion sent them scurrying back to their homes.

The reservoir at the burning fuel storage complex in the Christian-held Dora area, adjacent to Beirut port, blew up due to intense heat from nearby fuel tanks set ablaze in earlier attacks, authorities said.

The blast shattered windows, blew down doors and cracked walls within a 3-mile radius, and injured about 100 people, most of whom suffered cuts from flying glass.

"I thought it was an earthquake," a West Beirut housewife said. "We ran out of the house to the streets as people usually do at the time of tremors."

A huge pillar of black smoke billowed from the reservoir site, blocking the sun over Beirut.