The United States added Hamza bin Laden, son and heir of the late Osama bin Laden, to its terrorist blacklist on Thursday.

Hamza, who is in his mid-twenties, has become active as an Al-Qaeda propagandist since his father's death at the hands of US special forces in 2011.

According to letters found in the Navy SEAL raid on Osama's hideout in Pakistan, Hamza wrote to the Saudi-born Al-Qaeda leader asking to be trained to follow him.

A CIA analyst who examined the letters told AFP that in July 2009, when Hamza wrote the letter to his father's Abbottabad hideout, they had not seen each other for eight years.

But the elder bin Laden was making plans to send for his son, who was at the time under house arrest in Iran, and planned to groom him to become Al-Qaeda's leader.

Since Bin Laden's death, his Egyptian deputy Ayman al-Zawahiri has taken up the reins of the organisation, but Hamza has issued audio messages to supporters.

The State Department order lists Hamza bin Laden as a "Specially Designated Global Terrorist," meaning that US citizens are forbidden from dealing with him, and freezing any assets under US jurisdiction.

In July 2016,Hamza had threatened revenge against the United States for assassinating his father, according to an audio message posted online.

He promised to continue the global militant group’s fight against the United States and its allies in the 21-minute speech entitled “We Are All Osama,” according to the SITE Intelligence Group.