The army on Tuesday identified the man as Muhammad Ali Qasim Yaqub, alias Abu Sohaib al Makki, and said he had been "working directly under al Qaeda leaders along (the) Pakistan-Afghan border".
Makki's arrest was the first of what authorities said was a prominent militant since US special forces killed bin Laden on May 2 in a secret raid.
"He is a mid-level operative and has been very active in the region, but I can't say yet if he is 'huge' in the global scheme of things," said the official, adding intelligence agencies arrested him about a week ago in central Karachi.
"Nonetheless, he is a very good catch."
There was no immediate way of verifying Makki's rank within al Qaeda.
A military official in Islamabad said Makki was an "explosives expert". The official said he was "definitely" linked to the al Qaeda leadership but did not elaborate.
"He is still being interrogated and we hope to get more information from him," said the official.
A senior security official in Islamabad said Makki was between 35 and 40 years old and had been living with his three children and wife "for some time".
Military sources said that he came to Pakistan in 2001 and operated along Pakistan's border with Afghanistan.
"The whole Osama issue has been very embarrassing for us, and that is why we have significantly stepped up efforts to capture any militants that may be hiding here," said another military official.
"We have always been very serious on terrorism, but now we are moving against militants even more seriously. You will hopefully see more results soon."
Officials say militants generate funds through extortion and kidnapping in Karachi.
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