Students of the Garissa University College take shelter in a vehicle after fleeing from an attack by gunmen in Garissa, Kenya, April 2, 2015. [Photo/IC]

The death toll in the attack by Somali Islamists on a university in northeastern Kenya on Thursday has risen to 147 and the siege has ended, the country's disaster response agency said.

"UPDATE: 147 fatalities confirmed in the Garissa Attack," The Kenya National Disaster Operation Centre said on its official Twitter feed, adding that the operation had ended.

Kenya's Interior Minister Joseph Nkaissery previously told reporters in Garissa that about 500 out of 815 students were accounted for, while four al Shabaab fighters were killed.

At the same press conference in Garissa, Kenyan police chief Joseph Boinet said the east African country has introduced a dusk to dawn (6.30pm - 6.30am) curfew for four regions near Somalia border as a security precaution.

Kenya has offered a 20 million shilling ($215,000) reward for information leading to the arrest of Mohamed Mohamud, a man labelled "most wanted" and linked to an attack on a university campus on Thursday.

"Seen this man? Report to the security agencies through the numbers provided. #KaaChonjo #GarissaAttack," the Interior Ministry said on its official Twitter feed.

The ministry also showed a picture of Mohamud in a poster.

Al-Shabab gunmen attacked Garissa University College on Thursday, targeting Christians. Witnesses said the gunmen said they were from al-Shabab, a Somali extremist Islamic group, and the group has claimed responsibility for the attack.