The United States on Thursday offered up to $1m for information leading to the identification or whereabouts of Osama bin Laden's son.

Described by the US State Department as an "emerging leader" within al-Qaeda, Hamza bin Laden has been classified as a "terrorist" by the US since 2017.

He has released audio and video messages on the internet calling for attacks on Western capitals and has threatened revenge for his father's killing in 2011 by US forces, according to a State Department statement.

He has also threatened to target US citizens abroad and urged Saudi tribes to unite with al-Qaeda's Yemeni branch to fight against the Saudi state.

WANTED. Up to $1 million for information on Hamza bin Laden, an emerging al-Qa'ida leader. Hamza is son of Usama bin Laden and has threatened attacks against the United States and allies. Relocation possible. Submit a tip, get paid. https://t.co/LtBVhsrwTc #RFJ pic.twitter.com/gmx5hkoSzQ

— Rewards for Justice (@Rewards4Justice) February 28, 2019

Bin Laden was killed that year in a covert operation carried out in Abbottabad, Pakistan at a compound where the al-Qaeda leader was holed up.

Bin Laden claimed responsibility for the Sept. 11, 2001 attacks on New York's Twin Towers, as well as the attack that day on the Pentagon.

Those attacks were carried out using three passenger planes hijacked by al-Qaeda operatives. A fourth plane, bound for either the White House or the Capitol, crashed in a field in Pennsylvania after passengers attempted to retake it from the hijackers.

Nearly 3,000 people were killed that day, including citizens of roughly 77 countries.

Letters from Osama bin Laden seized from the Abbottabad compound indicated that he was grooming his son to replace him as leader of al-Qaeda.

Hamza bin Laden's current whereabouts are unknown and it is not clear whether he holds other citizenship.

"We do believe he's probably in the Afghan-Pakistan border and he'll cross into Iran. But he could be anywhere though in ... southcentral Asia," Michael Evanoff, the US assistant secretary for diplomatic security said.

Hamza bin Laden was designated a "global terrorist" by the US in January 2017, prohibiting US individuals from conducting transactions with him.