Adam Gadahn, in undated pictures released by the FBI. (Reuters)

A U.S.-born spokesman for al-Qaeda has been arrested, Pakistani intelligence officers said Sunday.

Adam Gadahn was arrested in recent days in the southern Pakistan port city of Karachi, two intelligence officers said. They spoke on condition of anonymity because they weren't authorized to release the information.

U.S. defence, intelligence and law enforcement officials could not immediately verify the report, but Patrick Rowan, the former top anti-terrorism official in the Justice Department, said, "If this is him, it's a big capture and a morale-booster."

Californian-born Gadahn, 31, has been on the FBI's top 10 most wanted terrorist list since 2004 and was charged with treason in 2006. The agency has a $1-million reward for his capture.

Gadahn was arrested in the sprawling southern metropolis of Karachi in recent days, two officers who took part in the operation said. A senior government official also confirmed the arrest, but said it happened Sunday. The discrepancy could not immediately be resolved.

Gadahn was being interrogated by Pakistani officials, the intelligence officials said. Pakistani agents and those from the CIA work closely on some operations in Pakistan, but it was unclear if any Americans were involved in the operation or interrogation.

Gadahn, whose nom de guerre is said to be "Azzam the American," posts videos and messages regularly on the internet, calling for the destruction of the West and for strikes against U.S. targets.

His most recent post was made Sunday, when he praised the U.S. army major charged with killing 13 in Fort Hood, Texas, last November.

Gadahn, who converted to Islam at 17, moved to Pakistan in 1998 where he attended al-Qaeda training camps and associated with leaders of the militant group.

Under American law, treason can carry the death penalty.

With files from The Associated Press