Hamza bin Laden, the son of former al-Qaeda leader Osama bin Laden, has been stripped of citizenship by Saudi Arabia.

The country's interior ministry announced the move in a statement after the US state department said on Thursday it was offering a reward of up to $1m (£760,000) for information leading "to the identification or location in any country" of Hamza, calling him a key al-Qaeda leader.

In the notice published in the kingdom's official gazette on Friday, the ministry said his citizenship was revoked via royal decree in November. There was no explanation why the order was only made public now.

His exact whereabouts have been unknown for a number of years, though various reports have speculated he may be living in Pakistan, Afghanistan, Syria or Iran.

Saudi-born Hamza is thought to be around 30 years old, and was named as a “specially designated global terrorist” by the US in January 2017. He has released audio and video messages calling for attacks against the US and its allies.

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His father was the mastermind behind the 9/11 terrorist attacks in the US and a host of other assaults against western interests. He was killed in a US military raid in Pakistan in May 2011.

The US state department said Hamza married the daughter of Mohamed Atta, the leader of the group of al-Qaeda hijackers on 9/11, and that letters seized from his father's compound in Pakistan indicated he was grooming his son to one day replace him as leader of the group.

Although al-Qaeda has orchestrated a long list of terror incidents across the world, it remains best known for the September 11 2001 attacks on the US, which killed almost 3,000 people and injured 6,000 more.

Members of the group hijacked four commercial aircraft – flying two into the World Trade Center in New York City and one into the Pentagon in Washington DC.

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The fourth plane was heading towards Washington, before it crashed near Shanksville, Pennsylvania, when passengers staged a revolt and overpowered the hijackers.