M By Feisal Omar
Colonel Ali Dalel Hirsi, a military officer, said U.S. citizen Malik John had surrendered to officials on the edge of Barawe, about 220km (135 miles) southwest from the capital Mogadishu.
"(The American was caught) this morning on the outskirts of Barawe town," Hirsi said.
Al Shabaab wants to topple Somalia's Western-backed government and has staged frequent bomb and gun attacks in the capital Mogadishu. The group has also hit targets in neighbouring Kenya since Nairobi joined African Union peacekeeping forces in Somalia.
Hirsi passed the phone to John, who told Reuters from captivity he was a 31-year-old man from Maryland in the United States and had been fighting for al Shabaab for about four years.
"I decided to leave them two months ago. I hated them because I found their ideology was totally wrong," John said, adding that he finally escaped the group six days ago.
John said he walked for days, eating bread and maize on the way. "I am physically ok now but not mentally," he said.
Somalia's government has been trying to persuade many al Shabaab fighters to defect by promising amnesties for fighters who leave the group.
John said he had heard about the Somali amnesty. "If possible I would like to return my home in Maryland," he added.
The United States, other Western powers and countries in the region see the fight against the al Qaeda-aligned group as a vital part of the battle to prevent Islamist militancy spreading in East Africa and beyond.
In recent months some of the fighters within al Shabaab have pledged allegiance to the Islamic State, which has strained relations between different factions within the group. (Reporting by Feisal Omar, writing by Drazen Jorgic; Editing by Toby Chopra)