Kenya launches probe as Shabaab leader confirms mall attack

US, British and Israeli agencies are helping Kenya investigate an attack claimed by Somali Islamist militants on a Nairobi shopping mall that killed at least 72 people and destroyed part of the complex, officials said on Wednesday.

President Uhuru Kenyatta said on Tuesday that troops had defeated the al Qaeda-linked al Shabaab group after a four-day siege at the shopping centre popular with prosperous Kenyans and foreigners. He declared three days of mourning.

The attack has highlighted the reach of al Shabaab and the capabilities of its crack unit which claimed responsibility for the bloodshed in the Westgate mall, confirming international fears that Somalia would remain a recruiting and training ground for militant Islam as long as it remained in turmoil.

The militants stormed the mall, known for its Western shops selling iPads and Nike shoes, in a hail of gunfire and grenades at lunchtime on Saturday.

Late on Wednesday night, al Shabaab's leader for the first time confirmed claims by his group's members that it was behind the attack on the mall.

In an audio posted on the al Shabaab-linked website www.somalimemo.net, Ahmed Godane, also known as Mukhtar Abu al-Zubayr, said the attack was in retaliation for Kenya's incursion in October 2011 into southern Somalia to crush the insurgents.

"Take your troops out or prepare for a long-lasting war, blood, destruction and evacuation," Godane said in the message delivered in the Somali language and apparently directed at the Kenyan government. Kenyan troops are fighting alongside African peacekeepers against the militants in Somalia.

Al Shabaab had threatened revenge since Kenyan troops joined the conflict, warning that they would bring the "flames of war" to a country that is east Africa's biggest economy. The group has created funding, recruiting and training networks in Kenya.

"You are part of the massacre Kenya carried out in Kismayu and in other towns because you had elected your politicians. The tax you pay is used to arm Uhuru (Kenyatta) forces that massacre Muslims. You had supported the fight against us," Godane said in the message apparently directed to Kenyans.