At least 54 people, including 13 children, were killed in two separate transport tragedies in western Pakistan on Sunday.

Forty-one are so far confirmed dead after their bus crashed into a ravine in southwestern Balochistan province, while at least 10 students died in the boating accident in northwestern Khyber Pakhtunkhwa, officials said.

As many as three are still missing in the waters, with a rescue operation underway.

At the remote site of the bus crash, north of the city of Bela in the Lasbela district, senior administration official Hamza Anjum said "the dead bodies ... are beyond recognition."

Anjum said 40 corpses were retrieved from the wreck alongside three injured, one of whom died shortly after. The remaining two survivors were in "serious" condition.

The charred brown husk of the vehicle chassis smoked on a dry riverbed under the bridge on Sunday, according to a video released by the provincial government.

A team of men used heavy machinery to move the twisted metal aside and pull out the burnt remains, which were then shrouded in white cloth.

Head of the local rescue service Asghar Ramazan told AFP the bus had been loaded with containers of oil.

"When the bus fell down, it immediately caught fire," he said. The oil "caused the fire to flare up so much that it was difficult to control," he added.

The bus was reportedly carrying a total of 48 passengers when it hit a pillar on the bridge and careened off course earlier on Sunday.

It had been traveling overnight between Balochistan's provincial capital of Quetta and the southern port city of Karachi.

"It is feared that the driver may have fallen asleep," Anjum said, also mentioning the possibility he had been speeding during the long-distance trip.

"We will investigate the causes of the accident," he said, adding that DNA tests would be needed to determine the identity of the remains, which had been "badly mutilated".

'Rescue underway'

In Khyber Pakhtunkhwa, local police official Mir Rauf told AFP all of the drowned so far recovered from the boating accident on Tanda Dam lake were aged between 7 and 14.

A total of 17 were rescued alive from the reservoir by Sunday afternoon.

"Everything was normal until suddenly the boat overturned," said 11-year-old survivor Muhammad Mustafa from his hospital bed in the nearby city of Kohat.

"I got stuck under the boat," he told AFP. "My shawl and sweater weighed me down, so I took them off."

"The water was extremely cold and my body went numb. I thought I was going to pass out when a man on an inflatable tube saved me."

One of the rescued was a teacher, who remained unconscious as the rescue operation continued for up to three pupils still missing.

The class of madrassa students "went out for a picnic and boating" at the scenic location, district police chief Abdul Rauf told AFP.

"According to the information so far, the boat was in a dilapidated condition and it was overloaded too," he said.

Ramshackle highways, lax safety measures and reckless driving contribute to Pakistan's dire road safety record.

Passenger buses are frequently crammed to capacity and seatbelts are not commonly worn, meaning high death tolls from single-vehicle accidents are common.

In November, 20 people, including 11 children, were killed when a minibus crashed into a deep and water-logged ditch in southern Pakistan.

According to World Health Organization estimates, more than 27,000 people were killed on Pakistan's roads in 2018.

Mass drownings are also common in Pakistan when aged and overloaded vessels lose their stability and pitch passengers into the water.

In July last year, at least 18 women drowned after an overloaded boat carrying about 100 members of the same family capsized during a marriage procession between two villages.