Somalia-based Al-Shabaab Islamists have vowed to launch more attacks in Kenya after the Garissa University siege that claimed 148 lives.

The African extremist group has launched frequent attacks in retaliation for Kenya's stationing of troops in Somalia.

In a statement, the al-Qaeda-inspired organisation said: "No amount of precaution or safety measures will be able to guarantee your safety, thwart another attack or prevent another bloodbath from occurring in your cities."

"Not only are you condoning your government's oppressive policies by failing to speak out against them, but are reinforcing their policies by electing them. You will, therefore, pay the price with your blood," the statement added.

According to reports, five militants were involved in the Garissa attack. One of them was arrested while attempting to flee while four others were killed in exchange of gunfire with security forces.

Investigators have also arrested several suspects, as an investigation is under way focusing on why early warnings were ignored by the police.

In another development, US President Barack Obama has promised not to cancel his scheduled trip to Kenya.

"We will stand hand-in-hand with the Kenyan government and people against the scourge of terrorism and in their efforts to bring communities together," the American leader said in a statement.

"Even at this difficult hour, the Kenyan people should know they have an unwavering friend and ally in the United States."