The four-day siege involving suspected Islamist militants at Nairobi's Westgate shopping centre is over, Kenya's President Uhuru Kenyatta says.Five attackers were shot dead by troops and 11 suspects are in custody, he said in a televised address to the nation.Kenya has "shamed and defeated our attackers" but the "losses are immense", he said, confirming that 61 civilians and six soldiers had died.Three days of national mourning have been declared starting on Wednesday.President Kenyatta said that several bodies lying under rubble from collapsed parts of the building were still to be recovered.Some 62 people remain in hospital. Many others are being treated for shock and are undergoing counselling."I promise that we shall have a full accountability for the mindless destruction, deaths, pain, loss and suffering we have all undergone as a national family."These cowards will meet justice, as will their accomplices and patrons, wherever they are," he said.Militants stormed the Westgate centre on Saturday, throwing grenades and firing on shoppers and staff.Somali Islamist group al-Shabab said it carried out the attack in retaliation for Kenyan army operations in Somalia.The president said he could not confirm reports that a British national and two or three US citizens were involved in the attacks, but he said forensic experts were carrying out tests to ascertain their nationalities.