ISLAMABAD, Pakistan -- At least 21 people died in freezing temperatures when tens of thousands of visitors thronged a Pakistani hill town to see unusually heavy snowfall, sparking a major traffic jam, authorities said Saturday.

Police reported that at least eight people had frozen to death in their cars, while it was not immediately clear if others may have died from asphyxiation after inhaling exhaust fumes in snow-bound vehicles.

Interior Minister Sheikh Rashid said the military had mobilised to clear roads and rescue thousands still trapped near Murree, around 70 kilometres (45 miles) northeast of the capital, Islamabad.

Video shared on social media showed cars packed bumper-to-bumper, with one-metre-high (three-foot) piles of snow on their roofs.

"The heavy snowfall caused a traffic jam and the closure of roads," Babar Khan, a tourist who was stranded for hours, told AFP by phone.

"Roads were also closed due to falling trees in many places."

The website of Pakistan's National Weather Forecasting Centre said heavy snowfall was expected in the area until Sunday afternoon, while Information Minister Fawad Chaudhry said "decades" of weather records had been broken in the last 48 hours.

For days, Pakistan's social media has been full of pictures and videos of people playing in the snow around Murree, a picturesque resort town built by the British in the 19th century as a sanatorium for its colonial troops.

The Punjab province chief minister's office said the surroundings had been declared a "disaster area" and urged people to stay away.

Prime Minister Imran Khan said he was shocked and upset at the tragedy.

"Unprecedented snowfall & rush of ppl proceeding without checking weather conditions caught district admin unprepared," he tweeted.

"Have ordered inquiry & putting in place strong regulation to ensure prevention of such tragedies."

- Traffic warning -

Authorities warned last weekend that too many vehicles were trying to enter Murree, but that failed to discourage hordes of daytrippers from the capital.

"It's not only the tourists, but the local population is also facing severe problems," Usman Abbasi, another stranded visitor, told AFP.

"Gas cylinders have run out and drinking water is not available in most areas -- it's either frozen or the water pipes have been damaged due to severe cold."

Pakistan's armed forces public relations department said people still stranded in Murree would be moved to five army relief camps already established.

Rescue operations were continuing with heavy machinery working "without any pause", it said, adding: "Where machinery can't reach, troops have been moved and they are clearing traffic and opening roads."

The town of around 30,000, at an altitude of 2,300 metres (7,500 feet), clings to the sides of steep hills and valleys and is serviced by narrow roads that are frequently clogged even in good weather.

Sheikh Rashid said residents had sheltered people trapped in the town and provided blankets and food to those they could reach on the outskirts.

Authorities said schools and government buildings had taken in those who could make it to the town from the clogged roads.

Rescue 1122, Pakistan's emergency service, released a list naming 21 people it said had been confirmed dead.

It included a policeman, his wife and their six children.

Hasaan Khawar, a spokesman for the Punjab government, said they had frozen to death inside their snow-covered car.

Rescue 1122 said another family, of five, was also among the dead.