A Somali girl who said she had been raped has been stoned to death in Somalia after being accused of adultery, a human rights group has said.

Amnesty International said in a press release on Friday that the victim, Aisha Ibrahim Duhulow, had been 13 years old - not 23 as earlier reports had suggested.

Duhulow was stoned to death on October 27 by dozens of men in a stadium packed with 1,000 spectators in the southern port city of Kismayo, Amnesty International and Somali media reported, citing witnesses.

The armed group in charge of Kismayo had accused her of adultery after she reported that three men had raped her, Amnesty said.

"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.

Initial local media reports said Duhulow was 23, but her father told Amnesty International she was 13.

The Islamic Courts took control of much of Somalia in 2006, triggering an intervention by neighbouring Ethiopia, which propped up the UN-backed transitional government and ousted the movement.
  
While the group's political leadership largely fled the country, the movement's military and youth wing, the al-Shaabab, switched to guerrilla warfare.

In recent months the group appears to have gained strength, taking over the port city of Kismayo, and dismantling pro-government roadblocks.