Osama bin Laden's son Sa'ad was probably killed by a US missile strike in Pakistan earlier this year, US National Public Radio has reported, citing US intelligence officials.

Sa'ad bin Laden, a prominent figure in the murky relationship between Iran and al Qaeda, was believed to have gone to Pakistan after spending a number of years under house arrest in Iran, US intelligence officials said in January.

A missile fired from a US drone aircraft is believed to have killed him sometime this year, NPR cited an unnamed US counter-terrorism official as saying on its website.

US intelligence agencies were "80 to 85 percent" certain Sa'ad bin Laden was dead, the official said, adding that he was not important enough to be targetted himself but was "in the wrong place at the wrong time".

It was not clear whether Sa'ad bin Laden was near his father when he died, NPR added.

The official described him as being active in al Qaeda, but not a major player. He was thought to be in his late twenties.

US forces have stepped up their drone attacks in Pakistan since last September, targetting Taliban and Al Qaeda-linked militants in areas bordering Afghanistan.

The United States has carried out close to 50 such air strikes since the beginning of last year, killing about 470 people, including many foreign militants, according to a tally of reports from Pakistani intelligence agents, district government officials and residents. - Reuters