Dozens of men stoned Aisha Ibrahim Duhulow to death Oct. 27 in a stadium packed with 1,000 spectators in the southern port city of Kismayo, Amnesty International and Somali media reported, citing witnesses.
The Islamic militia in charge of Kismayo had accused her of adultery after she reported that three men had raped her, the rights group said.
Initial local media reports said Duhulow was 23, but her father told Amnesty International she was 13. Some of the Somali journalists who first reported the killing later told Amnesty International that they had reported she was 23 based upon her physical appearance.
Calls to Somali government officials and the local administration in Kismayo rang unanswered Saturday.
"This child suffered a horrendous death at the behest of the armed opposition groups who currently control Kismayo," David Copeman, Amnesty International's Somalia campaigner, said in a statement Friday.
Somalia is among the world's most violent and impoverished countries. The nation of some 8 million people has not had a functioning government since warlords overthrew a dictator in 1991 then turned on each other.
A quarter of Somali children die before age 5; nearly every public institution has collapsed. Fighting is a daily occurrence, with violent deaths reported nearly every day.
Islamic militants with ties to al Qaeda have been battling the government and its Ethiopian allies since their combined forces pushed the Islamists from the capital in December 2006. Within weeks of being driven out, the Islamists launched an insurgency that has killed thousands of civilians.
In recent months, the militants appear to be gaining strength. The group has taken over the port of Kismayo, Somalia's third-largest city, and dismantled pro-government roadblocks. They also effectively closed the Mogadishu airport by threatening to attack any plane using it.