The Somali militant group al-Shabab has claimed responsibility for shootings inside a shopping centre in the Kenyan capital Nairobi that has left dozens dead, the BBC has learned.

Al-Shabab, which has links to al-Qaeda, said the attack was in response to Kenya's presence in Somalia, where its troops have been fighting the militants since 2011.

Eyewitness, Abdulaziz, - who escaped from the shopping mall - told reporters that he saw a gunman leaving the scene with the crowds.