UPDATED 9.46am: Kenyan troops have ended a day-long college campus siege in which at least 147 students were massacred by Somalia's Al-Shabab Islamist group.

"The operation at Garissa University College has ended, with all four terrorists killed," the national disaster operations centre said late on Thursday, with the attack lasting some 16 hours from before dawn until well after dark.

The masked gunmen began the assault before dawn, using grenades to blast open the gates of the university in the northeastern town of Garissa, near the lawless border with war-torn Somalia, before attacking students as they slept.

The al-qaeda-linked Al-Shabab claimed the pre-dawn attack, the same insurgents who carried out the Westgate shopping mall massacre in Nairobi in September 2013, when four gunmen slaughtered at least 67 people in a four-day bloodbath.

Kenya has been hit by a wave of grenade and gun attacks, often blamed on sympathisers of the Al-Shabab and sometimes aimed at police targets, since the army crossed into southern Somalia in 2011 to attack Islamist bases.