Kenyan authorities are holding eight people in connection with an attack by Islamist militants on a Nairobi shopping mall and have released three others after the assault that killed 67 civilians and soldiers, the interior minister said on Friday.

Somali Islamist group al Shabaab said Saturday's attack, which ended after a four-day siege by soldiers and police, was "just the premiere of Act 1" and suggested it would be followed by other actions by its "warriors".

As the police investigation advanced, a leading Muslim cleric urged security forces to avoid harassing Muslims in their pursuit of those responsible.