The United States is offering a reward of up to $1m (£750,000) for information about the son of the late al-Qaeda leader Osama Bin Laden, Hamza.
Hamza Bin Laden is emerging as a key leader of the Islamist militant group, the state department says.
In recent years, he has released audio and video messages calling on followers to attack the US and its Western allies in revenge for his father's killing.
US special forces killed Osama Bin Laden in Pakistan in 2011.
He approved the attacks on the US on 11 September 2001, in which nearly 3,000 people were killed.
Hamza Bin Laden, who is believed to be about 30 years old, was officially designated by the US as a global terrorist two years ago.
He married the daughter of Mohammed Atta, who hijacked one of the four commercial aircraft used in the 2001 attacks, and crashed it into one of the World Trade Center towers in New York, says a BBC report.
Letters from Osama Bin Laden seized from his compound in Abbottabad where he was killed indicated that he had been grooming Hamza, thought to be his favourite son, to replace him as leader of al-Qaeda, according to the state department.
"We do believe he's probably in the Afghan-Pakistan border [sic] and... he'll cross into Iran. But he could be anywhere though in... south central Asia," said Assistant Secretary for Diplomatic Security Michael Evanoff.
Hamza Bin Laden is believed to have spent years with his mother in Iran, where it is thought his wedding took place, while other reports suggest he may have lived in Pakistan, Afghanistan or Syria.
