The Canadian Prime Minister has promised a full investigation into how a runaway-train, carrying over 70 containers of crude oil, derailed and exploded killing at least five people in the province of Quebec.

Stephen Harper has described the centre of the town of Lac-Mégantic as resembling a war zone. At least 40 people are still unaccounted for.

Authorities are expecting the death toll to rise as fire crews tried to reach the hardest hit areas more than two days after the disaster.

The growing number of trains transporting crude oil in Canada and the US had raised concerns of a major derailment, and this one will add to the debate over a proposed cross-US oil pipeline that Canada said it badly needs.

All but one of the train's 73 tanker cars were carrying oil when they came loose, sped downhill nearly 11km into the town and began exploding one by one.

"This is an unbelievable disaster," said Mr Harper who toured the town and compared it to a war zone.

"This is an enormous area, 30 buildings just completely destroyed, for all intents and purposes incinerated. There isn't a family that is not affected by this."

The downtown bar area had been bustling at the time of the crash. Quebec provincial police Sgt Benoit Richard said only a small part of the devastated scene had been searched as firefighters made sure all flames were out.

About a third of the community of 6,000 was forced out of their homes.

Locals were convinced the death toll was far higher than five. Anne-Julie Huot, 27, said at least five friends and about 20 acquaintances remained unaccounted for.

"I have a friend who was smoking outside the bar when it happened, and she barely got away, so we can guess what happened to the people inside," Ms Huot said. "It's like a nightmare."

A coroner's spokeswoman said it may not be possible to recover some of the bodies because of the intensity of the blasts.

The train's oil was being transported from North Dakota's Bakken oil region to a refinery in New Brunswick. Because of limited pipeline capacity in the Bakken region and in Canada, oil producers are increasingly using railroads to transport oil to refineries.

The Canadian Railway Association recently estimated that as many as 140,000 carloads of crude oil will be shipped on Canada's tracks this year, which is up from 500 carloads in 2009.

The Quebec disaster is the fourth freight train accident in Canada under investigation involving crude oil shipments since the beginning of the year.

Mr Harper has called railroad transit "far more environmentally challenging", while trying to persuade the Obama administration to approve the controversial Keystone XL oil pipeline from Canada to the Gulf Coast.

Greenpeace Canada said that federal safety regulations have not kept up with the enormous growth in the shipment of oil by rail.

Officials with the Montreal, Maine & Atlantic Railway said that despite the disaster, they feel transporting oil by rail is safe.

"No matter what mode of transportation you are going to have incidents. That's been proven. This is an unfortunate incident," said Joe McGonigle, Montreal, Maine & Atlantic Railway's vice president of marketing.

He said the company believes the train's brakes were the cause. "The train was parked, it was tied up. The brakes were secured. Somehow it got loose," he said.

Mr McGonigle said there was no reason to suspect any criminal or terror-related activity.