Nairobi - Somalia's al-Shabaab Islamic extremists claimed responsibility Wednesday for an attack that killed four people in Kenya's volatile northern region, bringing to 12 the number of those killed by the militants since Friday.Kenyan officials said that four people were shot dead in a shop and their bodies burned in Wajir town on Tuesday night by unknown gunmen who wounded three others. Kenyan Interior Minister Joseph Nkaissery said the motive is unclear and the attackers are not known. But al-Shabaab through its radio station in Somalia said they were responsible."The attack is part of a series of attacks targeting the Northern Frontier District region under Kenya's occupation," said al-Shabaab's radio Andulus."The Mujahideen forces who carried out the attack safely returned to base." Kenya's northern and eastern region border Somalia and a significant number of inhabitants there are Kenyan-Somalis.Al-Shabaab has been attacking Kenya since Kenyan troops were deployed to Somalia in Oct 2011 to fight the militants.Police statistics show that 312 people have been killed in al-Shabaab attacks in Kenya from 2012 to 2014.On Sunday four people believed to be non-Muslims were killed in an attack by gunmen suspected to be al-Shabaab militants in Mandera town. On Friday the militants claimed responsibility for attacking the convoy of Mandera County Governor Ali Roba in which four people died.Al-Shabaab carried out large-scale attacks in Mandera last year. The militants hijacked a bus and singled out 28 non-Muslims forcing them to lie on the ground before shooting them dead. Ten days later the 36 non-Muslim quarry workers were killed by the extremists.On Wednesday four men - one Kenyan and three Somalis - were charged in a court in Nairobi with the killings of quarry workers.