* U.S. says its troops were present at checkpoint raid
* Islamists say one fighter killed in the incident
* U.S. acknowledges small number of troops in Somalia (Recasts, and adds comment from U.S. officials)
By Feisal Omar and Abdi Sheikh
MOGADISHU, March 9 (Reuters) - The United States said Somalia's army successfully conducted a mission to disrupt an illegal taxation checkpoint set up by al Shabaab Islamist militants south of the capital Mogadishu on Wednesday, and said American forces were present at the raid.
The United States acknowledges having a small number of troops deployed in Somalia but rarely discusses their activities.
No comment was immediately available from the Somali government.
Washington said it launched air strikes on a separate al Shabaab base on Saturday, killing more than 150 Islamist fighters, though the militants said the number was exaggerated.
"During the mission, our troops exited the aircraft and stayed in a safe area to observe the actions on the objective," said Colonel Mark Cheadle, an Africa Command spokesman, without offering additional details.
Two U.S. officials, speaking on condition of anonymity, said those U.S. troops were only acting in an advise and assist role.
Earlier on Wednesday, an al Shabaab spokesman said one of its fighters had been killed in a gun battle at about 1 a.m. (2300 GMT) in the Awdigle district of Lower Shabelle area, about 50 km (30 miles) south of Mogadishu.
Sheikh Abdiasis Abu Musab, al Shabaab's military operations spokesman, said the two helicopters landed on the banks of the River Shabelle and commandos from the aircraft advanced on the base.
"They were masked and spoke foreign languages which our fighters could not understand," Abu Musab told Reuters. "We do not know who they were but we foiled them."
He said the commandos carried rocket launchers and M16 rifles, referring to a weapon used by U.S. forces although Abu Musab did not mention any nationality.
Residents in the area also said there had been a gun battle and said they saw helicopters in the area. They said the mobile phone network did not work during fighting.
"We were awoken by exchange of heavy guns," resident Ahmed Farah said by phone. "We could see the helicopters land and fly."
He said al Shabaab later sealed the area so it was not possible to know if there were any casualties.
Al Shabaab wants to topple the Western-backed Somali government that is attempting to rebuild the country after two decades of civil war and lawlessness triggered by the overthrow of President Siad Barre in 1991.
The fragile government is being backed by international aid aimed at preventing the country from becoming a haven for al Qaeda-style militants in East Africa. (Additional reporting by Phil Stewart; Editing by Edith Honan and Louise Ireland)