Al-Qaeda has urged non-Muslims - especially in the US - to convert to Islam, according to a new videotape.
Gadahn is suspected of running al-Qaeda's propaganda operation
The call is made by a man identified on the film as "Azzam the American", a convert also known as Adam Gadahn who is wanted for questioning by the FBI.
He says ignorance of Islam leads Westerners to accept wars waged by their governments and Israel against Muslim countries.
The video opens with an introduction by al-Qaeda number two Ayman al-Zawahiri.
Separately, Palestinian militants who held two Fox News journalists hostage for almost two weeks last month vowed to target all non-Muslims who entered the Palestinian territories.
"Any infidel blood will have no sanctity," the Holy Jihad Brigades group said in a statement posted on the internet.
The group, which was previously unknown, had forced the captured journalists to convert to Islam at gunpoint, according to one of the released hostages.
In the al-Qaeda tape posted on an Islamic website, Zawahiri made a brief statement urging viewers to listen carefully to the message, entitled: "An Invitation to Islam".
The video is introduced by al-Qaeda deputy leader Zawahiri
The speaker in the new video was identified as Azzam the American and appeared in the video dressed in a turban and seated in front of a computer.
Gadahn is believed by US authorities to be running al-Qaeda's propaganda operation.
He is thought to have appeared with his face covered in a 2004 video shown by ABC News in which the speaker threatened al-Qaeda attacks.
"To Americans and the rest of Christendom we say, either repent [your] misguided ways and enter into the light of truth or keep your poison to yourself," he said in English on the new tape.
"But whatever you do don't attempt to spread your misery and misguidance to our lands," he added.
"Ignorance" of Islam "causes the people of the West to rapturously applaud when Israel perpetrates wholesale slaughter of Muslims in Lebanon and Palestine," he added.