WASHINGTON - Only one of the five people killed in the raid that got Osama bin Laden was armed and fired a shot, a senior U.S. defence official said Thursday, acknowledging the new account differs greatly from original administration portrayals of a chaotic, intense and prolonged firefight.

The sole shooter in the al Qaeda leader's Pakistani compound was quickly killed in the early minutes of the commando operation, details that have become clearer now that the Navy SEAL assault team has been debriefed, the official said, speaking on condition of anonymity because he was not authorized to speak on the record.

He said the raid should be described as a precision, floor-by-floor operation to hunt and find the al Qaeda leader and his protectors, rather than as it has been portrayed by a succession of Obama administration briefers since bin Laden's death was announced Sunday night.

As the Navy SEALs moved into bin Laden's compound, they were fired on by bin Laden's courier, who was in the guesthouse, the official said. The SEALs returned fire, and the courier was killed, along with a woman with him. The official said she was hit in the crossfire.

The Americans were never fired on again as they encountered and killed a man on the first floor and then bin Laden's son on a staircase, before arriving at bin Laden's room. Officials have said bin Laden was killed after he appeared to be lunging for a weapon.

White House and Defence Department and CIA officials through the week have offered varying and foggy versions of the operation, though the dominant focus was on a firefight that officials said consumed most of the 40-minute assault.

"There were many other people who were armed ... in the compound," White House spokesman Jay Carney said Tuesday when asked if bin Laden was armed. "There was a firefight."

"We expected a great deal of resistance and were met with a great deal of resistance," he said.

"For most of the period there, there was a firefight," a senior defence official told Pentagon reporters in a briefing Monday.

And though officials later revised these words, White House counterterrorism adviser John Brennan originally said even bin Laden, too, took part in the shootout. Later the administration said bin Laden wasn't armed.

NBC News, which was first to report that four of the five people killed were unarmed, said the majority of the operation was spent gathering up the compound's computers, hard drives, cellphones and other items that could provide valuable intelligence on al Qaeda and potential operations worldwide.

Those materials have been taken to the FBI lab at a Marine Corps base near Washington, the defence official said Thursday.