Kenya said Saturday it has arrested five men in connection with the massacre by Somalia's Al-Qaeda-linked Shebab militants of nearly 150 people at a northeastern university.

The militants meanwhile warned of "another bloodbath" and a "long, gruesome war" unless Kenya withdrew its troops from Somalia.

Thursday's attack on Garissa University, situated near the border with Somalia, left 148 dead, including 142 students, three police officers and three soldiers. It was Kenya's deadliest attack since the 1998 bombing of the US embassy in Nairobi, and the bloodiest ever assault by the Shebab militants.

Kenyan interior ministry spokesman, Mwenda Njoka, said five arrests had already been made.

"Three were the coordinators who were arrested while trying to flee to Somalia, two were arrested within the precincts of Garissa University," he told AFP, noting that the four gunmen in the university were killed on Thursday.

The name of the three suspected organisers were not given, but Njoka said the two arrested on campus included a security guard at the university, and a Tanzanian named as Rashid Charles Mberesero.

Mberesero was reportedly arrested on the campus on Friday, found hiding as people carried out the grim work of clearing the piles of bodies that the gunmen had killed.

"He was hiding in the ceiling of the university and had grenades," Njoka said, while the guard, a Kenyan of ethnic Somali origin, was named as Osman Ali Dagane.

READ: What's behind the return of al-Shabab, the terror group that killed at least 147 people in Kenya?

He is suspected of helping the gunmen, and was found "in possession of jihadist materials," Njoka added.

A $215,000 (200,000 euro) bounty has also been offered for alleged Shebab commander Mohamed Mohamud, a former Kenyan teacher believed to now be in Somalia and said to be the mastermind behind the Garissa attack.

Shebab threat


Hurling grenades and firing automatic rifles, the gunmen stormed the university at dawn on Thursday as students were sleeping, shooting dead dozens before setting Muslims free and holding Christians and others hostage.

Just before darkness fell, Kenyan troops moved in on the dormitory where the gunmen were holed up, apparently determined to prevent a drawn-out siege like that seen in the Westgate shopping mall in Nairobi in September 2013, also carried out by Shebab fighters.

The militants vowed more attacks against Kenya, which invaded southern Somalia in 2011 and is now fighting alongside the African Union force battling the Shebab.

"We will... stop at nothing to avenge the deaths of our Muslim brothers until your government ceases its oppression and until all Muslim lands are liberated from Kenyan occupation," the Shebab said in a statement released Saturday.

"Until then, Kenyan cities will run red with blood... this will be a long, gruesome war of which you, the Kenyan public, are its first casualties."

PHOTO GALLERY: Attack at Kenyan University

On Friday, United States embassy vehicles were seen entering the campus - in past attacks including the Westgate massacre, FBI investigators helped Kenyan police with forensic examinations.

"Forensics and investigations are still going on," Njoka said.

The Shebab on Friday described how they had specifically singled out non-Muslims to kill, gathering them together before executing them.


Warnings missed

Survivors who hid from the attack have recounted how the gunmen called on people to come out of their dormitory bedrooms and lie on the ground face down, but then killed them.

A photograph seen by AFP from inside the building showed over 50 students squeezed together lying down on the ground, all them apparently executed where they lay.

Survivors also recounted how the gunmen taunted students before killing them.

"The mujahedeen stormed the university compound and swiftly proceeded to the halls of residence where they had gathered all the occupants," the Shebab statement said.

"And since the attack targeted only non-Muslims, all Muslims were allowed to safely evacuate the premises before executing the disbelievers."

Emergency workers completed the grim task of collecting bodies on Friday, with the final toll declared as 148, with all the corpses flown to the capital Nairobi, where grieving families wait to reice the remains of their loved ones.

For the traumatised survivors still in Garissa, students on Saturday waited outside the university compound to collect their belongings before being taken to the capital Nairobi. The college has been ordered to be closed indefinitely.

There has been growing criticism in the media that critical intelligence warnings were missed.

Kenya's National Union of Teachers have called for extra security measures, warning advising "all teachers in northen Kenya to leave if they feel unsafe."