Somali Islamists have stoned to death a woman accused of adultery in the first such public killing by the militants for about two years, witnesses said.

The 23-year-old woman was placed in a hole up to her neck for the execution late on Monday in front of hundreds of people in a square of the southern port of Kismayu, which the Islamist insurgents captured in August.

Stones were hurled at her head and she was pulled out three times to see if she had died, witnesses said. When a relative and others surged forward, guards opened fire, killing a child.