Al Shabaab militants in Somalia publicly beheaded four men accused of spying for the country's Western-backed government, the United States and neighbouring Kenya, residents in the south of the Horn of Africa country said.

The al Qaeda-linked group confirmed the executions, which took place on Sunday after the men were found guilty by an al Shabaab court in Jamame district of lower Jubba region, some 70 km (43 miles) north of Kismayu.

"The court ruled on their cases and four of the men were executed publicly in Jamame District according to the Sharia this (Sunday) afternoon," Mohamed Abu Abdalla, al Shabaab's governor for the Jubba region, told Reuters, without elaborating on the method of execution.

"The four men admitted they were spies."

In areas under its control, al Shabaab has carried out executions, floggings and amputations after summary trials in cases ranging from espionage to theft.

Al Shabaab has been fighting for years to drive out African Union peacekeepers, topple the central government and impose its own harsh interpretation of Sharia (Islamic law) in country.