Osama bin Laden’s eldest grandson has died, according to a letter written by the boy’s father and obtained by terror monitors.

The former al-Qaeda leader’s son Hamza described Osama bin Hamza bin Laden, thought to have been aged 12, as a “hero cub” and “martyr” but gave no cause of death.

One report, citing jihadi sources, suggested the boy had been killed in an air strike on the Pakistan-Afghanistan border but others said he was more likely to have succumbed to an illness.

The letter, translated into English by al-Qaeda propaganda arm the Global Islamic Media Front, was addressed to the bin Laden family. It was posted on Twitter by terrorism cyber-monitor Jihadoscope.

“We condole ourselves and you on the martyrdom of our hero cub... the grandson of our bravery,” wrote Hamza bin Laden.

He said his son, named after the former al-Qaeda leader, ”often acted out the image of his martyrdom, lying on the ground, eyes closed and smiling slightly” as he played with other children. He added the boy had been “sad to a point we did not expect” at the death of his grandfather, the mastermind of the 9/11 attacks on New York’s World Trade Centre and the world’s most wanted terrorist until he was killed by US forces in 2011.

Saudi-owned news channel Al-Arabiya, citing sources familiar with the matter, said the boy’s death at some point last year was “most likely due to an illness he caught and not receiving proper medical attention”.

But Arabic news website Arabi 21 reported claims he had been killed in an air raid on an al-Qaeda base.