Nairobi - Three police officers were killed and two others injured when their vehicle ran over an improvised bomb near Kenya's eastern border with Somalia, a Kenyan official said on Wednesday.The police vehicle was moving from Kula Police Post toward the Liboi border town, North Eastern Regional Co-ordinator Mohamud Saleh said. He said three other officers have been unhurt.The incident came a day after police chief Joseph Boinnet announced al-Shabaab extremists were stepping up attacks on Kenya. Boinnet said al-Shabaab is under pressure from African Union troops supporting Somalia's weak government against al-Shabaab's insurgency.In the last two weeks, attacks by the group in Garissa and Mandera counties have increased after a lull. Last week, an improvised bomb killed four people in a vehicle, including a minor.Mandera County has been hardest hit in recent years by an al-Shabaab campaign to avenge Kenyan troop presence in Somalia since 2011.Kenya's troops are part of the African Union force in Somalia. Kenya has managed to stop the frequency of al-Shabaab attacks in its capital, Nairobi, and major towns, but human rights groups say the government uses methods such as extrajudicial killings that can fuel revenge attacks.