The tape, posted on an Islamist Web site, features Ayman al-Zawahiri, Osama bin Laden's top lieutenant, introducing "Azzam the American," a convert who, as Adam Gadahn is wanted by the FBI, the BBC reported.
"To Americans and the rest of Christendom we say, either repent misguided ways and enter into the light of truth or keep your poison to yourself," he said, speaking in English with Arabic subtitles. "But whatever you do don't attempt to spread your misery and misguidance to our lands."
Laura Mansfield, an expert on the Middle East who serves as a corporate consultant, said the videotape could signal a planned attack, since Muslims believe their infidel targets should be given a chance to convert.
Gadahn said even people working for the British and U.S. governments would be welcomed.
"We invite all Americans and believers to Islam, whatever their role and status in (President George W.) Bush and (Prime Minister Tony) Blair's world order," he said. "Decide today, because today could be your last day."