Media playback is unsupported on your device Media caption It is feared the crash could be Canada's worst rail disaster in 50 years

Investigators have begun gaining access to parts of a Quebec town levelled when a runaway crude oil train exploded on Saturday, police have said.

The death toll in Lac-Megantic has risen to 13, with about 37 more still missing, police have said.

Meanwhile, authorities warned it could be days before search teams could access the entire disaster zone.

At least 30 buildings were destroyed by the fireball, including a grocery store and the public library.

The crash could be Canada's worst rail disaster in 50 years, reports the BBC's Paul Adams.

The devastation was so great Canadian Prime Minister Stephen Harper said the town looked like "a warzone" as he toured the area on Sunday.

Hazardous conditions, including dozens of tanker wagons strewn about the site, continue to hamper search efforts.

On Monday afternoon, police spokesman Benoit Richard said at least 12 crime scene investigators were working alongside a major crimes unit and specialised forensics lab personnel.

He said the toll of the dead and missing - previously reported as 40 - had risen as more residents called authorities with reports.

"Hopefully we'll get through the whole scene in the next couple of days," he said.

'Nightmare'

A spokeswoman for Quebec's coroner's office, Genevieve Guilbault, pleaded with the families of the missing to provide DNA samples to investigators by bringing in toothbrushes, hairbrushes, combs, razors, hats or other items.

Many of those missing were believed to have been drinking at a popular downtown bar when the explosions occurred.

Anne-Julie Huot, 27, said at least 25 people she knew were still missing.

"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," she told the Associated Press news agency. "It's like a nightmare."

One man who escaped from the bar told CBC that he had been unable to sleep because he kept hearing the screams of his friends.

None of the dead have been identified. At least five of the bodies recovered so far have been sent to Montreal for identification.

Officials have warned that some of the dead may never be recovered, given the intensity of the fire.

Firefighters worked over the weekend to put out remaining fires.

On Monday, they were dousing two oil-filled cars with water and foam in an attempt to keep them from overheating and exploding.

Crews are also working to contain 100,000 litres (27,000 gallons) of crude oil that have spilled into nearby waterways.

A spokesman for Quebec's environmental ministry says floating barriers and other tools are being used to block the oil from heading downstream.

But the pollution has already reached the nearby town of Saint-Georges, prompting fears oil could flow into the St Lawrence River.

Brakes questioned

Media playback is unsupported on your device Media caption Eyewitnesses say the streets "were filled with fire"

The train had been parked in the village of Nantes, about 7km (four miles) from Lac-Megantic, during an overnight driver shift-change on Friday evening.

Firefighters from Nantes were later called to put out a small fire on the train, but it is not clear if that was linked to what happened next.

Sometime afterward, 73 cars carrying pressurised containers of crude oil came uncoupled from five locomotive engines, gathering speed as they rolled downhill before derailing in the heart of Lac-Megantic, about 250km east of Montreal.

An official statement from the train operator said the brakes on the locomotive had somehow completely shutdown after the engineer left the train.

This "may have resulted in the release of air brakes on the locomotive that was holding the train in place", the firm said.

The explosion sent a fireball and black smoke into the air, forcing the evacuation of 2,000 people. Of those, 1,500 are unlikely to return to their homes immediately, officials said.

Canada's transportation safety board has retrieved the train's data recorder and a separate device that contains details of the braking system.

The train had been travelling from the Bakken Field in North Dakota to a refinery in Saint John, New Brunswick.