Somali militants have killed 36 non-Muslim quarry workers near the north Kenyan town of Mandera according to report. The attackers from the al-Shabab group shot the non-Muslims dead after separating them from Muslims, residents said. There is growing concern in Kenya about security in regions bordering Somalia after a spate of attacks. Al-Shabab killed 28 people in an attack on a bus targeting non-Muslims in the same area last week. The attack on the quarry workers took place early on Tuesday. Witnesses said the victims were caught after midnight, while sleeping in their tents at the quarry in Kormey, 15km (nine miles) from Mandera town. A driver who visited the scene of the attack, Ali Sheikh Yusuf, told BBC that most of the victims appeared to have been lined-up, and shot in the head, at close range. He said four were beheaded inside their tents, while three appeared to have escaped to Mandera town. Al-Shabab said it carried out the attack, blaming the involvement of Kenyan forces in Somalia. The group put the number of people killed at 40, higher than official accounts. In response to the killings, President Uhuru Kenyatta has summoned senior security officials. Kenya’s Red Cross said that security personnel and one of its own teams were at the scene soon after the attack. Earlier, one person was killed in a bar popular with non-Muslims in a neighbouring district. On Monday night, gunmen reportedly opened fire and hurled grenades at the bar in the town of Wajir, killing one and injuring 12. Mandera County borders both Somalia and Ethiopia, and it is dominated by Somalis, who are largely Muslims. Many of the quarry workers killed are reported to have come from the south of the country where Christians predominate. Al-Shabab has stepped up its campaign in Kenya since 2011, when Kenya sent troops across the border to help battle the militants.