MOGADISHU, Somalia — Somali Islamist insurgent fighters beheaded seven people accused of abandoning their religion and of espionage, residents said Friday, in the largest mass execution since the Islamists were chased from power two and a half years ago.

The execution follows weeks of bloody fighting for control of the capital and at a time of mounting concern over the influx of hundreds of extremist foreign fighters into the war-ravaged Horn of Africa nation.

Resident Madey Doyow said he saw seven headless bodies in a truck being guarded by militia members in the southwestern town of Baidoa.

"The people who were guarding the vehicle told us they were beheaded for violating Islamic law," he said. The victims had come from different parts of the southwestern Bay and Bakool regions, he said.

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

She called the killing of her husband inhuman.

Hawa, a woman who also wanted her full name withheld, was at the police station in Baidoa along with other families who had been informed a relative was executed. She told the AP that her brother had been missing for about 20 days after being abducted from his house by masked men, and that she had just been informed that he had been beheaded.

Al-Shabab militia officials told her that the seven had been accused of either renouncing the Islamic religion or spying for the transitional government, she said.

The executions fit a broader pattern of torture and unlawful killings by various factions of the militia, said Benedicte Goderiaux, a Somalia researcher for Amnesty International.

In Kismayo, a southern port city under al-Shabab control, a 13-year-old girl was stoned to death last October after being accused of adultery and a man also had a hand amputated after being accused of theft.

Last month the militia amputated a hand and foot each from four men accused of theft in the capital of Mogadishu. Another man accused of rape and murder was stoned to death in a town south of the capital last month.

"It's difficult to know if the unlawful killings and torture is increasing or it's the reports that are increasing," said Goderiaux.

"It's definitely linked to al-Shabab wanting to show or portray themselves as restoring law and order in the region they control ... it's also linked to them wanting to terrorize the population under their control under the guise of applying sharia law,"

The killings come as African Union officials are deliberating over broadening the mandate of an under-resourced and undermanned peacekeeping mission in the country's capital, and as the U.N. human rights chief said both the insurgents and government troops may be committing war crimes.

Somalia has been a failed state for the last 18 years. The current U.N.-backed government, supported by 4,300 AU peacekeepers, is struggling to maintain its control of a few blocks of the capital. The Islamic insurgency, which seized much of the south and the capital for six months in 2006 before being driven from power, has been strengthened in recent months by an influx of weapons and fighters from other countries.

Associated Press Writer Katharine Houreld in Nairobi, Kenya contributed to this report.