US special operations forces working with African partners called in an air strike against the Al-Qaeda-linked Shebab group in Somalia on Thursday, killing five, the Pentagon said.

Pentagon spokesman Navy Captain Jeff Davis said US troops were advising and assisting Ugandan troops from the African Union Mission to Somalia (AMISOM) in southern Somalia, west of Mogadishu.

The AMISOM troops were raiding an illegal Shebab roadblock where the jihadists were extorting payments from drivers.

"They came under fire from the Al-Shebab militants, and we called in an air strike in their defense," Davis said.

A US defense official said the strike was conducted by drone. Five Shebab fighters were killed, and there were no reports of injuries to the Ugandan or US troops.

Another defense official had earlier said the US troops took part in the firefight, but Davis said that was not the case.

"We were nearby, but not directly involved," he said.

The Shebab group was chased out of the capital Mogadishu in 2011 but remains a dangerous threat in both Somalia and neighboring Kenya, where it carries out frequent attacks.

The United States has a small presence of about 50 troops, assisted by air power, in the impoverished country.

The Pentagon periodically announces results of its strikes in Somalia, including one in March on a Shebab training camp that killed more than 150 fighters who were planning a "large-scale" attack.

Davis said it is not unusual to see Shebab members setting up roadblocks.

"It's a very remote country with lots of big uninhabited areas where if there's a road, it's not hard for a bad guy to set up a spot there to be able to shake down people who go down the road," Davis said.

US special forces are working alongside local partners to fight jihadists in several countries across Africa and the Middle East.