US officials believe a son of al Qaeda ringleader Osama bin Laden has been killed in an American missile strike in Pakistan earlier this year.

According to National Public Radio (NPR), Saad bin Laden -- Osama's third-oldest son -- is said to have been killed by Hellfire missiles fired from a US Predator drone.

US spy agencies are "80 to 85 percent" sure that Saad bin Laden is dead, a senior counterterrorism official told the US broadcaster.

"We believe he is dead, although in all of these instances, unless you have DNA or other hard evidence, you don't know for sure," acknowledged the official.

Saad is believed to have fled Afghanistan shortly after the US invasion in October of 2001. According to media reports he went to Iran, where he was held under house arrest for a number of years.

Former US spy chief Mike McConnell said earlier that the younger bin Laden, supposedly in his 20s, is likely to have resettled in Pakistan.

Earlier in January, the US Treasury Department froze the assets of Saad bin Laden and three other al Qaeda operatives

According to authorities, the son was not considered a significant player in al Qaeda. A US official commented on his death report, saying that Saad bin Laden was simply "in the wrong place at the wrong time".

It is not known whether Saad was close to the location of his father -- who remains at large and is believed to be hiding in the violent tribal belt along the Afghanistan-Pakistan border -- when he was killed.