Osama bin Laden, the leader of al-Qaeda, has called on the people of Somalia to overthrow Sharif Sheikh Ahmed, their president.

Bin Laden made his comments in an audio recording that was posted on a website on Thursday.

He described Sheikh Ahmed as a person who disowns Islam.

As-Sahab, al-Qaeda's media company, released the tape entitled Fight on, champions of Somalia, in which Bin Laden said: "He [Sheikh Ahmed] must be dethroned and fought."

Bin Laden compared Sheikh Ahmed to Hamid Karzai, the Afghan president, Burhanuddin Rabbani, a former Afghan president, and Ahmed Shah Massoud, a prominent commander who fought during a 10-year war against Soviet occupation in Afghanistan.

"These sorts of presidents are the surrogates of our enemies and their authority is null and void," the Bin Laden said.

Bin Laden also called on Muslims to help Somalis topple the "apostate government".

It is his second message in less than a week.

The previous message accused Arab leaders of being "complicit" with Israel and the West against Muslims and urged them to liberate the Palestinian territories.

At least 60 messages have been broadcast by Bin Laden, Ayman al-Zawhiri, his second-in-command, and their allies since the attacks on the US on September 11, 2001.

The al-Qaeda leader has a $23 million US bounty on his head.