Yesterday a car bomb exploded outside a police station in the Kenyan capital, Nairobi, killing four people.

Police were taking the occupants of the car in for questioning in the Pangani neighbourhood when it exploded, the interior ministry said.

Two of those killed were police.

It is unclear who was behind the blast, but Nairobi has been targeted in the past in attacks attributed to Somali al-Shabab militants, who oppose Kenya's military involvement in Somalia.

A grenade also found at the scene was detonated by police, the ministry said.

Kenya has been on alert for attacks since last September's assault by al-Shabab gunmen on the Westgate shopping mall in Nairobi left 67 people dead.

Pangani district is near the Eastleigh suburb, where many people of Somali origin live.

The blast comes three weeks after police arrested more than 600 people in Eastleigh following explosions that killed six.

Most were later released without charge.

In March, police in the port city of Mombasa arrested two people driving a car with two improvised bombs hidden inside it.

Kenya sent troops to Somalia in 2011 to fight al-Shabab and the al-Qaeda-affiliated group vowed to retaliate by launching attacks in Kenya.

President Uhuru Kenyatta met ethnic Somali leaders in March to ask for help in identifying people they thought may be behind recent attacks in the capital.