Police officers have killed four suspected extremists in Kenya’s volatile Mandera County, a border region hard-hit by recent attacks by the Somali extremist group Al-Shabab.

Police killed the men at the Somali border while ambushing a group of about 20 suspected members of a cell of Al-Shabab militants on Saturday, North Eastern Regional Coordinator Mohamud Saleh said Sunday.

The group allegedly was planning an attack on a police station. Police recovered four rifles and Somali army jackets during the ambush, Saleh said.

Al-Shabab, the Al-Qaeda affiliate in East Africa, has vowed retribution on Kenya for its troop presence in Somalia.

Kenya deployed troops to Somalia in 2011 to fight Al-Shabab, which is waging an insurgency against Somalia’s weak UN-backed government and threatening instability in the region.

The extremist group has carried out at least five mass attacks in the last two years. Al-Shabab gunmen killed 18 people last month in separate attacks on non-Muslims in Mandera County, which lies in northeast Kenya and borders both Somalia and Ethiopia.

Group members killed the 28 non-Muslims on board a bus they hijacked in Mandera in November 2014 and the next month killed 36 quarry workers. This year, six people were killed in July when gunmen shot at buses.

Al-Shabab’s campaign targeting non-Muslims has had a devastating impact on education in Mandera. Many non-Muslim teachers have asked to be transferred from the region, creating a teacher shortage.

The Kenyan government announced last year it was building a wall across the nearly 700 kilometers (435 miles) or border it shares with Somalia to prevent the proliferation of people immigrating illegally, contraband and extremists.

But the construction stopped and the workers were withdrawn after the Somali government vehemently opposed the wall.

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