Nairobi Westgate attack: Who are the al-Shabab militants?

At least 62 people have been killed and more than 170 injured in an attack on a shopping centre in Kenya's capital Nairobi, which began on Saturday.

Between 10 and 15 attackers - thought to be militants from the Somali al-Shabab movement - were still inside the Westgate shopping centre on Monday morning.

Speaking on BBC Breakfast, security analyst Tim Ripley from Jane's Defence Weekly explained who the Islamist militants are.