Al Shabaab fell out with its former spokesman and deputy leader, Mukhtar Robow Abu Mansur, in 2013. He defected to the UN-backed government in August last year.
Al Shabaab has been fighting for years to try to topple Somalia's central government and rule the Horn of Africa country according to its own interpretation of Islamic law.
"If Mukhtar Robow thinks he can destroy Islamic sharia and the mujahedeen, he is deluded. Allah will protect Islam and Jihad will not stop just because of you and your likes who joined the enemies," Ali Mohamud Rage, al Shabaab's spokesman, said in a video posted late on Monday.
It was not immediately possible to reach Robow for comment.
"No doubt, Mukhtar Robow left his religion and joined the disbelievers and the enemies are still the enemies," al shabaab's spokesman said.
"Anybody who joins the line of non-Muslims is an apostate who can be killed."
A report by rights body Human Rights Watch released on Monday said al Shabaab had threatened and abducted civilians in Somalia’s Bay region to force communities to hand over their children for indoctrination and military training in recent months.
"Al Shabaab's ruthless recruitment campaign is taking rural children from their parents so they can serve this militant armed group," said Laetitia Bader, senior Africa researcher for the rights body.
The insurgents, who are allied with al Qaeda, were driven out of the capital Mogadishu in 2011. They have also since lost nearly all other territory they previously controlled after an offensive by Somali government troops and African Union-mandated AMISOM peacekeepers.
Al Shabaab, however, remains a formidable threat and has carried out bombings both in Mogadishu and other towns against military and civilian targets.
Mukhtar Robow Abu Mansur
Somali al Shabaab militants, who have carried out frequent bombings in the capital, Mogadishu, said a former leader who defected to the government side was an apostate who could be killed.