Karachi, Pakistan ? Two Pakistani officers and a government official said Sunday that an American charged with treason for working with al-Qaida had been captured, a development that could deliver another significant blow in the U.S.-led battle against the terror network.

U.S. defense, intelligence and law enforcement officials could not immediately verify the reported detention of Adam Gadahn, a 31-year-old spokesman for al-Qaida who has appeared on videos threatening the West, including one that emerged earlier Sunday.

The reported arrest of Gadahn follows the recent detention of several Afghan Taliban commanders in Karachi, including the group’s No. 2. Those detentions have been seen as a sign that Pakistan, which has been criticized as an untrustworthy ally, was cooperating more fully with Washington.

Some observers were cautious about giving credence to the claim that Gadahn was in custody as reports emerged that the man arrested might instead be a Taliban militant leader. There was no way of independently verifying the arrest or identity, and detentions of terror suspects in Pakistan are often surrounded by conflicting reports.

“If this is him, it’s a big capture and a morale-booster,” said Patrick Rowan, the former top anti-terrorism official in the Bush Justice Department.

Gadahn, the first American to face treason charges in more than 50 years, has appeared in more than half a dozen al-Qaida videos, taunting the West and calling for its destruction. The video that surfaced Sunday showed him urging American Muslims to attack their own country.

“It’s a blow to al-Qaida and a boost to the U.S. when a guy that has been taunting the U.S. for years has been captured,” Rowan said.

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.

They spoke on condition of anonymity because they were not authorized to release the information.

The intelligence officials said Gadahn was being interrogated by Pakistani officials. Pakistani agents and those from the CIA work closely on some operations in Pakistan, but it was not clear if any Americans were involved in the operation or questioning.

In the past, Pakistan has handed over some al-Qaida suspects arrested on its soil to the United States.

If the man in custody is indeed Gadahn and authorities can get him to talk, he could offer valuable intelligence about al-Qaida’s second in command, Ayman al-Zawahiri, and maybe even Osama bin Laden, Rowan said.

Gadahn has been on the FBI’s most wanted list since 2004 and there is a $1 million reward for information leading to his arrest. He was charged with treason in 2006 and faces the death penalty if convicted. He was also charged with two counts of providing material support to a designated foreign terrorist organization.

Gadahn grew up on a goat farm in Riverside County, Calif., and converted to Islam at a mosque in nearby Orange County.

He moved to Pakistan in 1998, according to the FBI, and is said to have attended an al-Qaida training camp six years later, serving as a translator and consultant. He is known by various aliases, including Yahya Majadin Adams and Azzam al-Amriki.