Uhuru Kenyatta, Kenya's president, has said the al-Shabab assault that killed 148 people at a university is an "attack on humanity" and vowed to take harsh measures against what he called "extremists".

Kenyatta also declared three days of national mourning on Saturday, in a televised speech delivered two days after four al-Shabab fighters stormed Garissa University College in the country's east and shot students in their dormitories.

"My administration shall respond in the severest way possible to the attack and any other attack to us," Kenyatta said.

"The time has come for us to be honest with ourselves and indeed each other. The radicalisation that breeds terrorism is not conducted in the bush at night. It occurs in the full glare of day in homes, in madrassas and in mosques with rogue imams."