Osama bin Laden's son Hamza is poised to lead a stronger, larger al-Qaeda and is 'bent on avenging' his father's death, according to a former FBI agent.

Washington: Osama bin Laden's son Hamza is poised to lead a stronger, larger al-Qaeda and is "bent on avenging" his father's death, according to a former FBI agent familiar with the personal letters seized in a dramatic US raid that killed the al-Qaeda leader in Pakistan's Abbottabad.

Hamza, about 28 year old now, wrote those letters when he was 22 and had not seen his father bin Laden in several years. Ali Soufan, the former FBI agent who was the bureau's lead investigator of al-Qaeda after the 9/11 attacks, told CBS News the letters reveal Hamza to be a young man who adores his father and wants to carry on his murderous ideology.

The letters collected in the raid by helicopter-borne US Navy SEALs at a secure high-walled compound in Abbottabad, a garrison town north of the Pakistani capital, Islamabad, in May 2011 have now been declassified.

Only a handful of US military and senior officials around then US President Barack Obama knew of the raid that was reportedly in the planning for months.

For a new episode of '60 Minutes' on the network, Soufan described one of those letters from Hamza: "He tells him that...he remembers 'every look...every smile you gave me, every word you told me.'"

Hamza also wrote this: "I consider myself to be forged in steel. The path of jihad for the sake of God is what we live."

Soufan said Hamza's potential as a leader was recognised years ago when he was still a boy. He was used in propaganda videos, sometimes holding a gun.

"He was a poster kid for the al-Qaeda...and for members of al-Qaeda, who were indoctrinated with these propaganda videos, he means a lot to them," Soufan said.

This January, the US named Hamza a "specially-designated global terrorist" - the same classification bin Laden had.

He even sounds like his father, said Soufan. "His recent message that came out, he delivered the speech as if it's his father...using sentences, terminology that was used by Osama bin Laden."

Hamza has recorded four audio messages in the last two years. "He's basically saying, 'American people, we're coming and you're going to feel it. And we're going to take revenge for what you did to my father...Iraq... Afghanistan'...the whole thing was about vengeance," Soufan said.