A Kenyan security forces vehicle was attacked with a makeshift bomb on Friday, wounding four, a day after the first serious clash between Kenyan soldiers and Somali militants inside the neighbouring nation, police said.

The attack is the fourth on Kenyan soil this week. Kenya sent soldiers and heavy weapons into Somalia 13 days ago to crush al Shabaab, the al Qaeda-linked militant group Nairobi blames for a string of kidnappings and border incursions.

Evidence of an improvised explosive device has been found ... We are treating the incident as a deliberate criminal act, police spokesman Eric Kiraithe said in a statement.

He said the vehicle was part of a convoy moving personnel to Nairobi from Liboi town on the border with Somalia. The attack took place 7 km from Garrissa town.

Two persons were arrested near the scene of the explosion and are still in police custody undergoing interrogation, Kiraithe said in the statement.

The Kenya Red Cross said the vehicle hit was in a convoy of four.

The vehicle in which the officers were travelling was extensively damaged and partly burnt, a police source in the provincial headquarters, who declined to be named, told Reuters.

Multiple sources told Reuters they were from the paramilitary General Service Unit (GSU), which comes under the umbrella of the police.

Four police officers were injured when the vehicle lost control due to the impact of the explosion, Kiraithe said.

NO REGRETS FOR GRENADE ATTACK

Two medical sources at the Garissa Provincial General Hospital, where the injured were taken initially, earlier said two of the GSU officers were in critical condition and a third had been admitted for treatment.

We have received three patients who have been injured. Two are in a serious condition with burns and fractures, said Musa Mohamed, a medical superintendent at the Garissa hospital.

Kiraithe said the four injured were later evacuated to Nairobi for treatment.

The attack came a day after al Shabaab gunmen ambushed Kenyan soldiers inside Somalia in the first serious clash since the east African country sent its troops across the border.

Two Kenyan soldiers were injured in the clash and one died on Friday, local media and military sources said, marking the first Kenyan battlefield casualty. The military did not respond to requests for confirmation of the death.

Five soldiers were killed when a helicopter crashed due to mechanical failure shortly after Kenya deployed its troops over the border.

Unknown gunmen also attacked a vehicle in the far north of Kenya, near the Somali border, on Wednesday, killing four government employees and wounding two guards.

On Monday, the capital Nairobi was hit by two separate grenade attacks on a bar and a bus terminus. One person was killed and 29 were wounded.

A Kenyan man who pleaded guilty to the grenade attack on the bus station and membership of al Shabaab was sentenced on Friday to life in prison by a Nairobi court.

Elgiva Bwire Oliacha, also known as Mohamed Seif, told reporters he had no regrets after a magistrate read out the life sentence for the grenade attack.

Earlier in the same court, prosecutors charged two more suspects with the same crimes as Oliacha. Omar Muchiri Athuman and Stephen Macharia Mwangi denied the charges and will appear in court again on November 4.

(Additional reporting by Humphrey Malalo and Duncan Miriri in Nairobi; Writing by David Clarke)