A 100-foot storage tower at a large paper mill in southwestern Virginia exploded in flames early yesterday morning, killing seven workers and injuring seven others in one of the worst industrial accidents in the state in seven years.

All the dead workers -- four men and three women -- were perched on scaffolds inside the tower, attempting to reline it with fiberglass and working with explosive chemicals, firefighters said.

The 3:20 a.m. fire at the Westvaco Co. plant in Covington -- about 200 miles southwest of Washington -- demolished all but 10 feet of the 100-foot high tower and apparently trapped the dead workers inside the eight-foot wide tower.

"We don't know the cause," said Westvaco spokesman Andy Dreszer. He said the dead workers were employed by a South Carolina firm that had been working at the plant for a week.

"The contractors were working with fiberglass and the chemicals involved are very hazardous," Dreszer said.

Officials in Richmond said they believed the death toll of seven persons was the largest in an industrial accident in Virginia since 14 persons were killed in 1973 in the collapse of a high rise under construction in Bailey's Crossroads in Northern Virginia.

At the height of the fire, which firefighters controlled within an hour, the blaze was visible for more than a mile in the mountainous community of 10,000 located near the West Virginia border. Westvaco, which employs 1,800 workers at the plant, is the region's main employer.

The victims were employes of International Reinforced Plastics of Denmark, S.C., and their bodies were seared by the fire. "All that remained were the trunks of the bodies," said Covington patrolman J.B. Hinkle, one of the first officers to arrive at the plant.

Seven other people suffered minor injuries fighting the fire and were released after being treated at a hospital there. The seven dead workers were the only people in the tower when it exploded.

The eight-foot wide tower is normally used to store wood pulp before it is bleached with chlorine dioxide to make paper.