What could have prompted the Nairobi attack?

Questions are being asked about what may have motivated the Nairobi shopping mall attackers.

As the hostage siege at a shopping centre in Kenya's capital Nairobi continues, questions are being asked about what the motives behind the attack might have been.

Neighbouring Somalia's al-Shabaab group have claimed responsibility for the attack that has so far killed at least 69 people, while more than 150 others have been injured.

Al Shabaab has said it carried out the attack because of Kenya's intervention in Somalia in 2011.

That sparked a series of attacks by the group in Kenya, which have tended to be small-scale incidents at small bars and local shops -- until Saturday.

Professor Joe Siracusa is the Associate Dean of Global Studies at Melbourne's RMIT University.

He's told Santilla Chingaipe about what he thinks could have motivated the group to carry out the attack.