American and Pakistani officials said the man arrested was Abu Yahya Mujahdeen Al-Adam, who was described as having been born in Pennsylvania and who was thought to be affiliated with the operations division of Al Qaeda, commanding fighters in Afghanistan.
Little else was known about him, American officials said, and it was not immediately clear that American officials were involved in the arrest.
Initial reports seemed to have confused the American with Adam Gadahn, a California native who has been a spokesman for Al Qaeda and often appears on videos calling for strikes against targets in the United States.
Mr. Gadahn has been on the F.B.I.’s most wanted list since 2004 with a $1 million reward for information leading to his capture, and he is the first American charged with treason in more than half a century. He is believed to have been operating on the border area between Pakistan and Afghanistan.
In a recent video, Mr. Gadahn urged Muslims to follow the example of Major Nidal Hasan, the American soldier charged with the shooting that killed 13 people in Fort Hood, Texas, in November.
But a senior Obama administration official in Washington said Sunday that he could not confirm reports that Mr. Gadahn had been captured.
While the importance of the arrest of the other American was not clear, it builds on the capture of several senior Afghan Taliban leaders in recent weeks, including Mullah Abdul Ghani Baradar, the No. 2 official in the Afghan Taliban leadership.
The senior Obama administration official said that Pakistani authorities have Mullah Baradar in custody and are still allowing American interrogators to question him regularly.
“He’s talking to us but we’re still in the trust-building phase,” the official said, speaking on condition of anonymity because the interrogation results are confidential. “He’s not giving us any actionable intelligence.”
The official said that Pakistani authorities are likely to have more leverage over Mr. Baradar than American officials, because of the longstanding relationship between the Taliban and Pakistan’s main spy agency, the Inter-Services Intelligence Directorate. American officials also assume that the I.S.I. tends to some of Mr. Baradar’s family members.
The officials discounted the likelihood that Pakistani authorities are using harsh interrogation tactics against Mr. Baradar. “They know what he knows,” the official said.
The arrest of Mr. Baradar and some half dozen other senior Afghan Taliban leaders in Pakistan in recent weeks have prompted some analysts to declare that the Pakistani intelligence service has committed itself to a new path to work against the Afghan Taliban, its long-time proxy against Indian interests in Afghanistan.
But the senior administration official voiced skepticism that there had been any strategic shift by the highest levels of the Pakistan spy agency.
“It’s still not clear what’s going on, but we haven’t concluded there’s some major shift,” the official said. “One theory is that this was a confluence of tactical operations.”
Another theory, he said, is that Mr. Baradar and the other captured Taliban leaders were purged by hardliners in the Afghan Taliban leadership, who grew distrustful of them.
A Pakistani high court ruled this month that Mr. Baradar could not be transferred to the Afghan government, despite efforts by the Afghans to seek his extradition. Moving Mr. Baradar or any other Taliban or Qaeda figure from Pakistani custody to Afghanistan would be tantamount to a handover to the United States, American officials said. Eric Schmitt reported from Washington; Mark Mazzetti contributed reporting from New York. via