Gadahn, one of the most wanted persons in the United States, was captured in the southern Pakistani port city of Karachi, Pakistani security officials were quoted as saying.

    Gadahn, 31, has appeared several times on videos threatening the West. A U.S. court charged the America-born al-Qaida spokesman with treason in 2006, making him the first American to face such a charge in more than 50 years.

    Earlier on Sunday, a video was released on a website showing the al-Qaida leader calling on Muslims in the United States to launch attacks against the country and undermine the U.S. economy.

    He called for "carefully timed and targeted attacks" to undermine the West's struggling economies, according to the SITE Intelligence Group, which monitors al-Qaida announcements.

    However, news of his detention cannot be verified independently, said security officials on condition of anonymity.

    Gadahn grew up in California and converted to Islam before he moved to Pakistan in 1998 and attended an al-Qaida training camp six years later, according to media reports.

    U.S. intelligence agencies had offered a 1-million-U.S.-dollar reward for information leading to Gadahn's arrest.