The SEAL team member's version of the Al-Qaeda chief's death contradicts previous accounts released by US President Barack Obama's administration and will stir debate on the handling of state secrets in the wake of the killing.

"No Easy Day", to be released next week, offers a first eyewitness account of the May 2011 raid, and recounts how a Navy SEAL sat on Bin Laden's body in the cramped Blackhawk helicopter that flew out of Pakistan after the operation.

Previous official accounts said Bin Laden had appeared in a doorway and ducked back into his bedroom, leading the US commandos to suspect he might be retrieving a weapon and thus to decide to use lethal force to stop him.

But the author, writing under the pseudonym "Mark Owen," said Bin Laden was shot in the head by the SEAL team when he leaned out of a doorway and was found bleeding from his wound when commandos made their way to his room, according to excerpts cited in media reports and confirmed to AFP by defense officials.

Bin Laden was mortally wounded and twitching on the floor as two women cried over his body. The team pushed aside the women and then fired more shots at the Al-Qaeda leader, according to the book.

"(We) fired several rounds," he says. "The bullets tore into him, slamming his body into the floor until he was motionless."

The book's publisher, Penguin Group's Dutton, has moved up the scheduled release date from September 11 to September 4, as media coverage has fueled a flood of orders for the book.

The Obama administration had said Bin Laden's body was treated with dignity and given a Muslim burial at sea.

But on the helicopter flight out of Pakistan, a SEAL member sat on Bin Laden's chest due to cramped quarters on the chopper, according to the book.

US officials insisted there was no display of disrespect, even if the author's description was accurate.

The commandos had already lost one helicopter in the operation, which crash landed at the compound, and this made for crowded conditions on the remaining aircraft, said a defense official.

"There's very little room, they were in a situation where they lost one bird. It was crowded," the official, who spoke on condition of anonymity, told AFP.

Special operations forces crammed into helicopters have sometimes had to sit on the bodies of their own fallen comrades in other raids, the official added.

Top officials from intelligence agencies and US Special Operations Command, which oversees the Navy SEALS, are reviewing the book to determine if the author revealed any classified information or secret tactics, which would violate Pentagon rules and trigger legal action.

"The Department of Defense has a copy of the book and we're taking a look at it," said spokesman Lieutenant Colonel Steve Warren.

The head of Special Operations Command, Admiral William McRaven, recently warned the elite unit that members could face criminal prosecution if they revealed information that endangered US troops, even if they were retired.

"I am becoming increasingly concerned about how former members of the special operations community are using their 'celebrity' status to advance their personal or professional agendas," McRaven wrote in the August 23 memo.

Fox News has revealed what it says is the identity of the author, a former Navy SEAL, who also took part in the 2009 operation that rescued Captain Richard Phillips from Somali pirates.

Obama administration officials appeared anxious to avoid having to defend and discuss again an operation they deem a major success, while suggesting the book did not shed any new light on the raid.

"We are not going to rehash the whole thing," said US defense official. "We're not going to comment on his account." The author credits Obama for endorsing the nighttime raid, but says his teammates made sarcastic remarks that the operation would help the president get re-elected.

The author, who writes under the pseudonym Mark Owen, told CBS television's "60 Minutes" program that he had wanted the book released on September 11 to avoid any political connotations during an election year.

"My worry from the beginning is, you know, it's a political season. This book is not political whatsoever. It doesn't bad mouth either party, and we specifically chose September 11th to keep it out of the politics," he told CBS, according to excerpts of the interview.

"You know, if these -- crazies on either side of the aisle want to make it political, shame on them. This is a book about September 11th, and it needs to rest on September 11th, not be brought into the political arena, because this -- this has nothing to do with politics," he said.

"None of us were huge fans of Obama. We respected him as the commander in chief of the military and for giving us the green light on the mission," he wrote. When one commando jokes again that they had ensured Obama's re-election, the author says, "Well, would you rather not have done this?"