The militants from neighboring Somalia attacked the settlement of Kamuthe in Garissa county, setting fire to a police post and destroying a telecommunication smast, police said in a report seen by The Associated Press.

Three non-Muslim teachers were killed and a Muslim one was abducted, the report said. The attackers spared the life of a female nurse due to her gender, it added.

Al-Shabab has vowed retribution on Kenya for its troops being present in Somalia since 2011 to fight the militants. The group has carried out numerous attacks on Kenya, killing hundreds.

Earlier this month, the al-Qaida-linked group attacked a military base used by Kenyan and U.S. forces, killing three Americans and destroying aircraft and other machinery.

Since December, al-Shabab has increased the frequency of attacks in the five frontline Kenyan counties that border Somalia and that the government has named as a hotspot for extremism. Security forces have been caught on the back foot each time, according to analysts.

On Jan. 2 al-Shabab claimed responsibility for killing four people in an attack on a convoy of buses and vans being escorted by police in Lamu.

The group killed 11 people in Mandera county, eight of them police officers returning to duty, who were pulled from a bus on Dec. 6. The group killed two non-Muslim construction workers in Garissa county on Dec. 17 and on Dec. 22 burned construction equipment owned by a Muslim contractor that was being used to develop a road in Mandera. On Dec. 27, two security officers died and seven other policemen were wounded when a roadside bomb tore through their patrol vehicle in Wajir county.