Five people have been arrested in connection with the attack on a Kenyan university that killed at least 148 people, according to Kenyan Interior Minister Joseph Nkaissery.

Masked al-Shabaab gunmen strapped with explosives stormed the Garissa University College campus, around 200km from the Somali border, in the pre-dawn attack on Thursday.

Tossing grenades and spraying bullets at cowering students, the attackers initially killed indiscriminately.

But they later freed some Muslims and instead targeted Christian students during a siege that lasted about 15 hours.

Anger over the massacre was compounded by the fact there were warnings last week that an attack on a university was imminent.

Local residents accused the authorities of doing little to boost security in this little-developed region.

Five people have been arrested in connection with the attack, CNN reported, citing Kenyan Interior Minister Joseph Nkaissery.

At least 79 people were wounded, many critically. But with an uncertain number of students and staff still missing, the casualties may yet mount.

"Yes, there is a likelihood of numbers going up," said one government source dealing with the Garissa attack.

Kenya's biggest-selling Daily Nation newspaper, citing sources, said the death toll would be significantly higher.

Outside the university gates, a throng of veiled women clung to the hope that missing people would still turn up alive.

One woman looking for her cousin who worked as a clerk at the university and has been missing since Thursday, said: "We are here waiting for news if we can find him, dead or alive."

The violence will heap further pressure on President Uhur Kenyatta, who has struggled to stop frequent militant gun and grenade attacks that have dented Kenya's image abroad and brought the country's vital tourism industry to its knees.

Yesterday, US President Barack Obama called Mr Kenyatta to express condolences over the "heinous terrorist attack" and confirmed he still planned to visit the country later this year, the White House said.

More than 400 people have been killed by al-Qaeda-allied al-Shabaab in the east African nation since Mr Kenyatta took office in April 2013, including the 67 people who died in an attack on a shopping mall in the capital Nairobi in September of that year.

Al-Qaeda itself killed around 207 people when it blew up the US embassy in Nairobi in 1998, an attack which remains the single biggest loss of life in Kenya since its independence from Britain in 1963.

Al-Shabaab says its recent wave of attacks are retribution for Kenya sending troops into Somalia to fight the group alongside other African Union peacekeepers.

The group, which at one point controlled most of Somalia, has lost swathes of territory in recent years but diplomats have repeatedly warned this has not diminished al-Shabaab's ability to stage guerrilla-style attacks at home and abroad.

Survivors of the Garissa attack spoke of merciless executions by the attackers, who stalked classrooms and dormitories hunting for non-Muslim students.

One survivor said he saw three female students kneeling in front of the gunmen, begging for mercy.

"The mistake they made was to say 'Jesus, please save us', because that is when they were immediately shot," he said.

Many students fled into the sandy scrubland, scaling barbed-wire fences and jumping off buildings, often half-naked, as they were awoken by the sound of gunfire and explosions.

"The attackers were just in the next room. I heard them ask people whether they were Christian or Muslim, then I heard gunshots and screams," said one student, who broke her hip when she jumped out of the first floor window of her dorm.

Within hours of the attack, Kenya put up a 20 million shillings ($215,000) reward for the arrest of Mohamed Mohamud, a former Garissa teacher labelled "Most Wanted" in a government poster and linked by Kenyan media to two separate al-Shabaab attacks in the neighbouring Mandera region last year.

The government also imposed a dusk-to-dawn curfew on Garissa, Mandera and two other regions near the porous border with Somalia.

However, diplomats and analysts say the move effectively concedes the government cannot control those areas, widely seen as Kenya's soft underbelly.