Iran publicly acknowledged for the first time on Wednesday that it was holding some senior figures from Osama bin Laden's al Qaeda network.
"Since the collapse of the Taliban regime we have arrested a large number of them [al-Qaeda members[," Iran's Intelligence Minister Ali Yunesi told reporters after a weekly Cabinet meeting.
"Many of them have been expelled and a large number of them are in our custody -- a mixture of big and small members."
He declined to identify any of the al-Qaeda members currently held in Iran.
"But I said big and small members," he reiterated.
It was the first public admission by a top government official that Iran is holding some key members of al-Qaeda, the group Washington holds responsible for the Sept. 11, 2001, hijacked plane attacks on US cities.
Previously Iran has said it was still trying to identify which al-Qaeda members it had captured.
The admission came two days after US President George W. Bush increased pressure on Iran by accusing it of harboring and assisting terrorists.
In strongly worded comments at the White House he said: "This behavior is completely unacceptable and states that support terror will be held accountable."
White House spokesman Scott McClellan said on Wednesday he was unable to confirm Yunesi's information and added Washington was "not exactly sure" what the Iranians meant by the term "custody."
But he said Tehran's statement appeared to confirm "what we believe to be a significant al-Qaeda presence in Iran."
Iran has flatly denied it harbored al-Qaeda members.
ABC News reported that the CIA had confirmed that al-Qaeda's security chief, Saif al-Adel, and the network's spokesman, Sulaiman Abu Ghaith, are in Iranian custody. A CIA spokesman had no comment on the ABC report.
Media reports and intelligence sources have also said Iran is believed to be holding al Qaeda's number two, Ayman al-Zawahri.