A Kenyan soldier with the African Union force in Somalia has killed six people, all believed to be civilians, after firing on them, a spokesman for the Kenyan Defence Forces (KDF) has said.

"Several people approached a KDF defensive position where a KDF soldier opened fire, killing six people," the army said in a statement on Monday.

"The soldier was immediately disarmed and has since been put on guarded seclusion."

The incident took place on Sunday in Jana Cabdalla, a town about 40km from the port city of Kismayo, the last major bastion of al-Shabab, the armed Islamist group.

Removing al-Shabab from Kismayo has been a long-term goal of Kenyan forces ever since they entered Somalia last October, but reaching the city has taken longer than initially planned.