MOGADISHU: A prominent US-born Islamist militant was killed in Somalia on Thursday after he fell out with senior commanders of the Al-Shabaab rebel group, witnesses said.

Residents in Al-Baate village in southern Somalia said Alabama-born Omar Hammami, commonly known as Abu Mansoor Al-Amriki or 'the American', and a British national known as Usama al-Britani, were shot dead in a dawn raid on their hideout.

Hammami's killing exposed widening rifts in Al-Shabaab's top ranks as the group affiliated to Al Qaeda grapples with an African Union-led military offensive that has captured key cities from the militants, depriving them of revenues.

“This morning Al-Amriki and his comrades were attacked by well armed fighters,” said village resident Hussein Nur. “After a brief fight al-Amriki and his two colleagues were killed. Several of their guards escaped.”

A second villager confirmed the gun battle and said he had heard Al-Shabaab fighters confirm the deaths, though he had not seen the bodies.

“No-one is allowed to go near the scene,” said a shopkeeper by telephone from the village that is under militant control.

Hammami is believed to have arrived in Somalia aged 22 in late 2006, shortly before a US-backed Ethiopian military incursion into the war-shattered Horn of Africa country to rout an Islamist administration that had dislodged the government.

Fluent in Somali, Hammami swiftly became an influential leader of Al-Shabaab's foreign militants fighting to topple a government seen as a Western puppet, and impose a strict interpretation of sharia law on Somalia.

Al-Shabaab announced a formal alliance with Al Qaeda in February 2012. Hammami was added to the US Federal Bureau of Investigation's Most Wanted Terror list in November that year and a $5 million bounty was offered for information leading to his capture.