A natural gas pipeline apparently pierced by construction workers exploded Friday in a huge pillar of flames that killed as many as 15 people and injured more than 100, crushing buildings, scorching cars and burning farm fields for hundreds of yards.

Officials feared the death toll could rise because some of the injured were in very critical condition.

The earthshaking blast occurred about 8:30 a.m. in an industrial miles southeast of Brussels (search), the capital, and could be seen and heard for miles. Emergency crews from across Belgium and from France, Luxembourg and Germany rushed to help, and a gray haze hung over the rural area as helicopters and about 50 ambulances carried injured to hospitals.

About a half hour before the explosion, construction workers alerted firefighters that they had damaged the underground gas line, acting provincial Gov. Guy Petit said. At least five of the dead and many of the injured were believed to have been police officers and firefighters responding to the call.

A towering wall of orange flames roared after the blast sent a series of huge fire balls boiling high into the sky. The shock wave crushed a swath of large buildings in an industrial park and hurled bodies more than 100 yards. Everything within 400 yards of the crater ripped open by the blast was melted or incinerated.

"There were bodies in parking lots, in fields. There were burned-out cars," said a spokesman for the Brussels fire department, Francis Boileau. "We have not seen devastation on such a scale for 40 years."

A local fire official, Jean-Claude Mondo, said cars were incinerated more than 100 yards away. The head of the local volunteer fire department, Eddy Pettiaux, was among the dead, Mondo said.

Prime Minister Guy Verhofstadt (search), who flew back from a vacation in Italy, said 15 people were confirmed dead and three were unaccounted for. He said 120 people were hospitalized, including 12 sent to burn centers in northern France.

Verhofstadt refused to speculate on what caused the accident, saying that would have to wait for an official investigation.

He said it was the worst such accident in Belgium since 1967, when a tanker truck carrying liquefied gas exploded and killed 22 people.

"The accident constitutes for our country a national catastrophe, and the toll is a heavy one," Verhofstadt told reporters in Ath, 6 miles from the blast site. He ordered flags to be flown at half staff and declared a national day of mourning when the victims are buried.

Officials said there could be more deaths among the gravely injured.

"There are many serious burn victims, with serious burns mostly on their backs," said Dr. Jean Francois Breckx at a military hospital in Neder-Over-Heembeek (search), a suburb of Brussels. "There are some 20 or so that are fighting for their lives."

People in nearby towns said they heard a deafening sound and felt the ground shake.

"It was like an earthquake," Michele Monvoisin, who lives about a half mile from the site, told The Associated Press. "There was a column of flames several hundred meters [yards] high, an enormous column of fire and black smoke. There was lots of debris falling on all the cars in our village."

In Ath, Carine Vanus told RTL television: "I had the impression that a plane was going to crash on the house. When we got out on the street, we saw a billow of white smoke and the sound was deafening."

People were advised to stay indoors with their windows and doors shut to keep out the heavy smoke hanging over the area, but the Health Ministry said it was not toxic and no evacuation was ordered.

The E429 motorway, which links Brussels with Lille in northern France, was closed, although officials said there was no danger of further explosions.