Rome: Late Al Qaeda leader Osama bin Laden's youngest son has urged Muslims to liberate the holy city of Jerusalem in an audio message, US terrorist-tracking organisation SITE tweeted on Monday.

"Jerusalem is a bride whose blood is our dowry," Hamza bin Laden purportedly says in the online message, quoted by SITE.

"The fight to free Jerusalem is a battle between faith and unbelief that requires the unity of the Ummah (the Muslim community)," SITE quotes the message as continuing.

Believed to be 23 or 24 years old, Hamza bin Laden has been touted as a future leader of the terror network.

"The liberation of Palestinian land has been brought much closer by the Syrian revolution," says the message, which SITE's director Rita Katz claims was recorded about five months ago.

The message was released a day after al Qaeda leader, the Egyptian-born Ayman al-Zawahiri allegedly urged rival militant groups in Syria to unite or risk death in an audio recording posted to the Internet.

In the audiotape, Zawahiri allegedly lent his support to the al Qaeda affiliated Al Nusra Front's plan to create a sovereign state in Syria that would rival the self-declared caliphate of the Islamic State jihadist group.

Zawahiri become al Qaeda's leader in June 2011, a few weeks after Osama bin Laden was killed in a raid on his compound in Pakistan by US special forces.

It is believed that Hamza bin Laden managed to escape from the compound during the raid by US navy seals. His current whereabouts are unknown.

Al Qaeda released a message in August, purportedly from Hamza bin Laden calling for Jihad against America and its allies. It specifically suggests London, Washington, Paris and Tel Aviv as targets for attacks.

He is believed to have taken part in attacks in Pakistan and Afghanistan as early as in 2005.