While some speculate whether he is dead or alive, Islamist terror leader Osama bin Laden's 50th birthday is believed to be marked on Saturday.

Supporters of bin Laden flooded websites with messages of praise and allegiance to the elusive leader and his al Qaeda network on Saturday.

For example, one Islamist website posted this message: "Today ... Osama bin Laden turns 50. God protect our leader, our Sheik Osama bin Laden. God reward him for his words and actions."

However, it's not known if Saturday marks the actual day of his birth. The FBI only lists that he was born in 1957.

The Saudi Arabia-born bin Laden's whereabouts remain unknown, more than five years after his disappearance after the Sept. 11, 2001 attacks. One of his final public appearances was in Jalabad, Afghanistan in late November of that year. He is believed to have escaped into neighbouring Pakistan.

Bin Laden is very familiar with that area. He helped the Afghan mujahedeen fight the Soviet army in the 1980s. In 1996, he moved to Afghanistan, then governed by the Taliban, where al Qaeda set up a series of training camps.

The manhunt has been fierce but little information on his whereabouts has been reported despite a US$25 million reward for his capture.

Rumours of his death have run rampant but one expert believes they are unfounded.

"Yes he is still alive. He's been communicating every now and again, usually about two to three messages a year for the past three years," analyst John Thompson told Â鶹´«Ã½net on Saturday.

Many think bin Laden is hiding in the rugged, isolated countryside of the North West Frontier area of Pakistan where Islamist tribal leaders can protect him.

"The real smart money is that he's not actually in the area where everybody is looking for him," Thompson said. "There is another school of opinion among intelligence analysts that he's actually a house guest in Iran.  That's the last public appearance he's said to have made in February of 2003."

Speculation that he is in Iran is further fuelled by recent sightings of his oldest son within the country.

The last time bin Laden appeared in a video tape, on Oct. 29, 2004, raised more questions over his whereabouts due to the high quality of the tape.

"The last television appearance Osama bin Laden made was not in a cave or a hillside up in the mountains but was in a professional television studio and that argues he was well inside another country and not in some wild, untamed frontier area," Thompson said.

Subsequent messages have been audio tapes. Ayman al-Zawahiri, his deputy, has been more active in speaking on behalf of al Qaeda.

Bin Laden, the son of a Saudi Arabian billionaire with ties to the Saudi royal family, founded the Islamic extremist network al Qaeda in 1988.

A spokesman for U.S. troops in Afghanistan expressed disgust over the celebrations.

"Instead of focusing on the anniversary of his birth, people around the world -- and particularly the people here in Afghanistan -- should take a moment to remember the innocent people who have been killed or injured by terrorist extremists like Osama bin Laden," said Maj. William Mitchell.

With files from the Associated Press