The mountain road covers open countryside, moorland and hills

Two aged 15 and two aged 16, were pronounced dead at the scene. Police said they were from the area.

They were in a Vauxhall Corsa car which went off the B4560, close to the Brecon Beacons National Park, at around 2100 GMT on Thursday.

Two men were taken to hospital. Their injuries were not life-threatening.

Police are appealing for witnesses to the crash on the Llangynidr to Beaufort road just outside the village of Garnlydan.

No other vehicle was involved. The road is likely to remain closed until at least midday.

A spokesman for the Welsh ambulance service said they were alerted at about 2110 GMT.

"We were on the scene fairly quickly and when we got there, we discovered it was a one-vehicle road traffic collision," he said.

"The two males were walking when we got there and they were taken to Nevill Hall Hospital in Abergavenny."

The road goes through open sections of country as well as moorland and hills.

The crash is said to have happened on a high section of the road at a location that has no safety barriers.

Roger Williams, Liberal Democrat MP for Brecon and Radnorshire described it as a "difficult" and "exposed" stretch of road.

He said it crossed an open common and there were often animals near the roadside.

"It's one of those roads that, if we do get some severe weather, it is closed early on," he said.

"With winter coming suddenly this year, perhaps people weren't ready for the difficult conditions."

As police were dealing with the fatal crash near Garnlydan, they were called to another about five miles away on the road between Crickhowell and Gilwern on Friday morning.

A 4x4 vehicle was recovered from the scene. The roads in the area were reported to be icy.