A mock movie poster warning of al-Qaeda's return to New York has appeared on several Arabic-language websites, but US authorities said there was no evidence of a genuine threat.

The image features a shot of New York at sunset with "Al Qaeda" in bold type followed by "Coming soon again in New York."

NYPD spokesman Paul Browne said the graphic was originally posted in the "artwork and design" section of a website that allegedly has close connections to Taliban forces and is used as a forum for terrorism.

The image has since been posted on other forums and websites.

The New York Police Department (NYPD) and the FBI were investigating where the image had come from.

The FBI was also exploring the origins and authenticity of the post, but added there was no real threat perceived.

Peter Donald, spokesman for the FBI's New York field office, said: "The FBI takes all threats seriously and at this time there is no specific or credible threat to New York."

The finding of the image comes as several al-Qaeda web forums were shut down in an alleged cyber attack.

The Washington Post reported that the group's main website, Shumukh al-Islam, a primary source for al-Qaeda videos and messages, went offline on 22 March. Four more have gone down since.