A United States-based human rights watchdog has demanded an independent investigation into the aerial bombing of a religious school (madrassa) in Pakistan's tribal Bajur agency.

"The attack on October 30 in the town of Khar had killed 82 people, including several children, though a military spokesman claimed the dead were all militants and denied any collateral damage," Human Rights Watch said.

Pakistani authorities say the seminary was being used as a training camp for Taliban and al-Qaeda militants, and that they issued a warning before striking.

However, Human Rights Watch quoted local residents as saying that only students were present in the seminary and that they received no warning. They also said the strike was carried out by fixed-wing US drones, which fired hellfire missiles as locals gathered to offer the dawn Fajr prayer.

"The Pakistani government should allow independent investigators into the area to determine who carried out the attack, how it was planned and executed, and who was killed," said Ali Dayan Hasan, South Asia researcher for Human Rights Watch.

"The onus is on the Pakistani government to provide a credible account of the legitimacy of the attack resulting in the deaths of so many," it said.

"If the government fails to provide a credible account of what happened, it will undermine legitimate anti-terrorist efforts," Hasan said adding Pakistan should allow investigators into the area to determine what happened.

ABC News quoted Pakistani intelligence officials as saying that the US carried out the attack. However, both US and Pakistani authorities denied the report. 

Pakistan's military spokesperson General Shaukat Sultan initially said that the attack was carried out on the basis of US military intelligence, but later retracted his statement, Human Rights Watch noted.

Asking the Pakistan government to justify the legality of the attack, it said: "If it was a law enforcement operation, the scale of the deaths point to use of excessive force in the extreme, with no or little effort to minimize loss of life. If it was a full-scale military operation, it raises real concerns about the proportionality of the attack and whether the attack was indiscriminate," it added.