“A total of four people have been injured. Three are administration police,” a Kenyan paramilitary police unit, a police officer in Garissa told AFP on condition of anonymity.
“A grenade was hurled on their vehicle” while the officers were on patrol, he added.
Regional police chief Philip Tuimur confirmed the incident but refused to give further details.
“We’re gathering information on what happened in the incident,” he said.
The attack comes after five police were killed in Garissa in November: two officers shot down while on patrol and three others killed by gunshots a few days later while changing a flat tyre on their vehicle.
Kenya has been hit by a wave of grenade and gun attacks, often blamed on sympathisers of Somalia’s shabaab Islamist fighters and sometimes aimed at police targets, since its army went into Somalia last year to flush out the Shabaab.