Hamza bin Laden, son and would-be heir of late al-Qaeda leader Osama bin Laden, has urged Muslims around the world to join the fight in Syria against "crusaders" and Shia Muslims.

"The cause of Syria is the cause of the entire worldwide Muslim community," he said in an undated audio recording released on militant networks on Thursday.

"In order for the people of Syria to resist the crusader, Shia and international aggression, Muslims - all Muslims - must stand with them, support them and give them victory," he said.

"Wakefulness is essential, as is quick, serious and organised movement, to support the people of blessed Syria before it is too late."

Hamza, who is in his mid-20s, has become more active as an al-Qaeda supporter since his father's death at the hands of US special forces in May 2011.

In new audio statement, Hamza Bin Laden stresses the importance of the jihad in #Syria; urges patience, unity & strength against US/#Russia: pic.twitter.com/LOL6FIDe1l — Charles Lister (@Charles_Lister) September 14, 2017

US intelligence officials have said that Hamza was the favourite son of the 9/11 mastermind, who had been grooming him to take over as al-Qaeda's leader.

Syria has been devastated by a six-year war that created an opening for militants, including the Islamic State (IS) group and the Fateh al-Sham Front.

Hayat Tahrir al-Sham, the Sunni group that last month seized control of the northern Syrian city of Idlib, is dominated by the al-Qaeda offshoot, which officially broke ties with the network founded by Osama bin Laden.

But experts say the name change was little more than a re-branding.

Many believe the group is positioning itself as more moderate than IS in hopes of a resurgence.

The United States added Hamza bin Laden to its terrorist blacklist in January.

The US Treasury estimates that he was born in 1989 in the Saudi city of Jeddah. His mother was Khairiah Sabar, one of the al-Qaeda founder's three wives.

Last year, experts began to note the son's increasing prominence in the movement.

The State Department has designated him a "Specially Designated Global Terrorist," freezing any assets he holds in areas under US jurisdiction.

In an undated audio message released in August 2016, Hamza bin Laden urged his Saudi supporters to rebel and overthrow the kingdom's rulers.

In that recording, Hamza urged Saudi youth to join the Yemen-based al-Qaeda in the Arabian Peninsula (AQAP) to "gain the necessary experience" to fight, according to SITE.

Experts believe he is preparing to take over the leadership of al-Qaeda and exploit IS defeats in Syria and Iraq to try to unify a global militant movement under the banner of al-Qaeda.