NAIROBI, Kenya — Somali Islamist fighters on Friday beheaded seven prisoners accused of abandoning the Muslim faith and spying for the government in the largest mass execution since the Islamists were pushed from power two and a half years ago.

The public killings in the southwestern town of Baidoa followed weeks of fierce fighting as the Islamists try to seize Somalia's capital, Mogadishu, amid mounting concerns about the influx of hundreds of foreign fighters to the failed state.

The beheadings may be linked to the Islamists' failure to take Mogadishu after a 2-month-old offensive, said a senior analyst at global intelligence company Stratfor.

"Al-Shabab is reacting to a setback," said Mark Schroeder.

The U.S. considers al-Shabab a terrorist group with links to al-Qaida, which al-Shabab denies. The group controls much of Somalia and its fighters operate openly in the capital.

Last month, the Obama administration announced that it would bolster efforts to support Somalia's embattled government by providing money for weapons and helping the military in neighboring Djibouti train Somali forces. An administration review of U.S. policy toward Somalia found an urgent need to supply the Somali government with ammunition and weapons as it struggles to confront increasingly powerful Islamic militants.

Government troops and African Union peacekeepers only hold a few blocks of Mogadishu, but they still control key government buildings as well as the port and airport, allowing them to receive arms shipments.

Schroeder said elements of the insurgency, always prone to splits, may have wanted to make a public show of strength after the failure to decisively capture Mogadishu.

Somalia's mostly clan-based militias frequently switch sides in the chaotic violence — the current president and the head of the insurgency were allies two years ago — and there have been signs of a power struggle in the area where the executions occurred.

Baidoa resident Madey Doyow, who spoke to members of the al-Shabab militia guarding the seven headless bodies, said the gunmen told him some of the executed men had links to pro-government militias.

The victims' frightened, weeping sisters and wives arrived at the police station in Baidoa on Friday to collect the mutilated bodies.

A woman named Miriam, who asked that her last name be withheld to protect her from reprisals, sobbed as she said in a telephone call that four bodies, including her husband's, had been brought to the police station. The location of the other three was unclear.