A French man who is suspected of being a key militant working for al-Qaeda has been arrested close to the border with Iran, officials say.
Naamen Meziche was detained during a security operation in the area.
He is described as being an associate of senior al-Qaeda leader Younis al-Mauritani, who was detained in 2011 accused of planning attacks on Europe.
Born in 1970 and of Algerian descent, Meziche is an "important" al-Qaeda member in Europe, experts say.
He is believed to have belonged to the Hamburg cell that the US says masterminded the 9/11 attacks.
Meziche reportedly recruited jihadists at a radical mosque in the northern German city, which authorities closed in 2010 because they said it was encouraging fanaticism.
Three of the 9/11 hijackers, including ringleader Mohammed Atta - who piloted the first plane into New York's World Trade Centre - met regularly at the mosque before moving to the US.
A Pakistani official told the AFP news agency that Meziche was "among the very close associates" of Mauritani who was himself arrested on 5 September 2011 by Pakistani agents - believed to have been co-operating with the CIA - in the city of Quetta.
Mauritani was believed to have been ordered by Osama Bin Laden - before his death last year - to plan attacks on Australia, Europe and the United States.
Correspondents say that Meziche's arrest highlights the key role played by the Pakistani security forces in the anti al-Qaeda campaign of the US, even though Washington and Islamabad are going through one of the rockiest stages in their relationship since the 9/11 attacks.
Tensions are also high because of continuing Pakistani resentment over the US decision to kill Bin Laden in May 2011 without telling Pakistan.
They have become further strained over US drone strikes on Pakistani soil and Islamabad's refusal to re-open a Nato supply route to Afghanistan which it closed down in November after 24 of its soldiers were killed on the border in a Nato air strike.
Neither Meziche nor Mauritani feature on the US FBI list of most wanted terrorists.
Earlier this month the White House said that Abu Yahya al-Libi, the "general manager" of al-Qaeda, had been killed in a drone strike in north-western Pakistan.