US intelligence agents in pursuit of al-Qaeda leader Osama bin Laden are now monitoring websites for messages to his followers.

Counter-terrorism experts believe they have found markers or code words that indicate bin Laden has been attempting to signal to supporters that he is alive.

One intelligence officer said: "It's either bin Laden or an elaborate cyber-deception campaign by his lieutenants."

Authorities are also investigating information from detainees that suggests al-Qaeda members - and possibly even bin Laden himself - are using steganography to hide messages inside photographic files on pornographic websites.

And it has been reported that US defence secretary Donald Rumsfeld has confirmed that bin Laden, not heard of since December, "has good reason not to make further public appearances".

"If he does, he will get caught," Rumsfeld said.

But reports have also suggested that if bin Laden is still alive, he might be waiting to emerge only when another attack occurs.

Last week, Abdel-Bari Atwan, the editor of London-based Arabic-language magazine al-Quds al-Arabi, said that he had been informed by associates of bin Laden that the al-Qaeda leader was alive and recovering from a shrapnel wound to his shoulder.

"They never indicated where he is," Atwan said.

While some intelligence officials believe that bin Laden could be hiding in the border area between Afghanistan and Pakistan, they have concluded that no one knows whether or not he is still alive.