Osama bin Laden's son Sa'ad, a prominent figure in the murky relationship between Iran and al Qaeda, has left that country and is believed to be in Pakistan, US intelligence said.

'The person you are talking about (Sa'ad bin Laden) has left Iran. He's not there. He's probably in Pakistan,' director of National Intelligence Michael McConnell told reporters.

Sa'ad bin Laden, who is believed to have played a role in several al Qaeda attacks, was reported to have been living in Iran since the US-led invasion of Afghanistan in 2001 that ousted the Taliban government shielding al Qaeda.

He is believed to have been under house arrest in recent years, but some reports say he also may have acted as an intermediary with Iran. The US government lists him as having been born in 1982.

The United States and Saudi Arabia have sought the repatriation of al Qaeda members in Iran.

A US counterterrorism official described the younger bin Laden as 'a very well-connected apprentice terrorist.'

McConnell declined to say whether there was any deal that led to Sa'ad bin Laden's relocation, and the counterterrorism official said the circumstances of his departure from Iran were unclear.

But McConnell suggested bin Laden's relocation to Pakistan was promising for US counterterrorism efforts. 'It's better for my world if any of these players are in places that we have access,' he said.

US officials have spoken of a growing success in crippling al Qaeda's central leadership through pressure on the Pakistan border areas with Afghanistan, where the elder bin Laden and top deputies are believed to be hiding.

Washington has accused Iran of harboring al Qaeda members, and the U.S. commission investigating the Sept.11 attacks said Iran and al Qaeda worked together in the 1990s.

But Iran has also arrested several activists, including the younger bin Laden, and al Qaeda, a militant Sunni Islamist group, often criticizes predominantly Shi'ite Iran.

Speculation on Iran's motives for holding the al Qaeda members includes having them available for surrender to the United States in exchange for nonaggression assurances, or as leverage to keep al Qaeda from acting against Iran's interests.

The US Treasury Department said earlier on Friday it would freeze assets of the younger bin Laden and three other al Qaeda members believed to be in Iran.

It said bin Laden, arrested in Iran in early 2003, may have left custody as of last September.-Reuters