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.
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.
Emerged in Afghanistan in the late 1980s, as Arab volunteers joined US-backed Afghan mujahideen fighting to expel the occupying Soviet forces
Osama Bin Laden set up an organisation to help the volunteers, which became known as al-Qaeda, or “the base”
He left Afghanistan in 1989, returning in 1996 to run military training camps for thousands of foreign Muslims
Al-Qaeda declared “holy war” on Americans, Jews and their allies
What has happened to al-Qaeda?
The US-led war in Afghanistan following the 2001 attacks toppled the Taliban regime which had given Osama Bin Laden and his group sanctuary.
In recent years, al-Qaeda was eclipsed by the Islamic State (IS) group which attracted global attention, fighters and funds, and carried out a number of attacks on Western targets and allies.
“Al-Qaeda during this period has been relatively quiet, but that is a strategic pause, not a surrender,” said US Co-ordinator for Counter-terrorism Nathan Sales.
“Today’s al-Qaeda is not stagnant. It’s rebuilding and it continues to threaten the United States and its allies…. Make no mistake, al-Qaeda retains both the capability and the intent to hit us.”