The speaker was identified as Azzam the American, also known as Adam Yahiye Gadahn - an Islamic convert from California wanted for questioning by the FBI and who US authorities believe is involved in a propaganda campaign for al-Qaeda.
"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 and suffer the consequences in this world and the next," Gadahn said in English.
He appeared in the video dressed in a white turban and seated in front of a computer and books.
"But whatever you do don't attempt to spread you misery and misguidance to our lands," he said.
Al-Qaeda's second-in-command Ayman al-Zawahri made a brief statement at the beginning of the tape - dated September 2006 - urging viewers to listen carefully to the message, entitled: "An Invitation to Islam".
Zawahri last appeared in a video in August in which he said that some leaders of Egypt's Gama'a Islamiya have joined al-Qaeda. Gama'a Islamiya later denied his statement.