A British Islamist and a fellow American extremist known as "the jihadi rapper" have been executed by their former terrorist partners in Somalia.

Osama al-Britani, a British citizen of Pakistani origin and Omar Hammami, known as Abu Mansur al-Amriki or "the American", have been killed by al-Shabab fighters in Gedo, a village south-west of the capital Mogadishu.

"There was a gun battle between Amriki and his men and other fighters, the reports are that Amriki is among those killed," local resident Moalim Ali told France 24.

A senior source within al-Shabab confirmed the deaths.

Alabama native Amriki, 28, was one of the most prominent foreign fighters to have joined the Somali jihadist group.

The son of a Syrian Muslim father and a Baptist mother, who he once described as a "typical Southern protestant girl", Amriki came to international prominence when he posted a series of Youtube videos in which he rapped about jihad.

His first 'hit', Blow by Blow, was released in 2009 and featured the repeated refrain: "Bomb by bomb, blast by blast, only gonna bring back the glorious past."

The FBI placed him on its most wanted list and the US State Department offered a $5 million bounty for his capture.

Last year Amriki parted from al-Shabab and founded his own militia after a quarrel with the group's leadership, who accused him of being selfish and loving the limelight.

"The opinions expressed by Abu Mansur ... are merely the results of personal grievances that stem purely from a narcissistic pursuit of fame and are far removed from the reality on the ground," al-Shabab said.

In April, Amriki claimed to have survived a murder attempt by a "al-Shabab assassin."

Amriki and his British companion, who is less well known, were allies of Sheikh Hassan Dahir Aweys, a senior Somali Islamist who also recently split from al-Shabab.