The United States is offering a reward of up to $1m (£750,000) for information about one of the sons of the late al-Qaeda leader Osama Bin Laden.
Hamza Bin Laden is emerging as a leader of the Islamist militant group, officials say.
He is thought to be based near the Afghan-Pakistani border.
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.
In 2011, US special forces killed Osama Bin Laden in a compound in Abbottabad, Pakistan. He approved the attacks on the US on 11 September 2001, in which nearly 3,000 people were killed.
What is known about him?
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.
The US state department says 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.
Letters from Osama Bin Laden seized from his compound indicated that he had been grooming Hamza, thought to be his favourite son, to replace him as leader of al-Qaeda.
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.
"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.
***