A blast struck a paramilitary convoy in eastern Kenya on Friday, seriously wounding three security officers .

The explosion happened just outside the town of Garissa on a road that leads to Kenya's massive Dadaab refugee complex and continues to the Somali border about 150 kilometers away.

Authorities were investigating if a land mine or bomb caused the explosion and who is responsible for the attack.

Attacks on Kenyan soil have increased since its military crossed into southern Somalia two weeks ago to hunt down al-Shabab militants. Officials believe the al-Qaida-linked group carried out several cross-border kidnappings in recent weeks.

On Friday a Kenyan court sentenced a man to life in prison for one of two grenade attacks that took place in the capital, Nairobi, on Monday.

Earlier this week, Elgiva Bwire Oliacha confessed to being a member of al-Shabab and taking part to an attack at a bus station that killed one and injured 20.

On Thursday, an al-Shabab militant leader in Somalia called on followers to carry out huge explosions inside Kenya.

Sheikh Muktar Robow says tossing grenades is not enough, and urges the militants to strike what he calls “big, painful blows.”