Military officials in Pakistan said Thursday that 23 militants have been killed in an exchange of fire with security forces in North Waziristan. The clashes started after a suicide bombing at a fort and checkpost of the region. A little-known group "Ansar-ul-Mujahideen " had claimed responsibility for the attack. The security forces launched a counter-attack and killed 23 militants, military officials said. "Terrorists tried to ambush a convoy of security forces which was returning back from Khajori post in the region," the officials said. "This convoy had gone for rescue of injured soldiers who were attacked by a suicide bomber yesterday when they were offering prayer in a mosque at the post." Officials said the security forces valiantly responded and encircled fleeing terrorists and inflicted heavy casualties on them. "Firefight continued for some time, which resulted into killing of 23 militants." Three security forces personnel were also injured in the exchange of fire, they said, adding at a search operation is in progress in the area. Tribesmen in the area said some mortars landed on a hotel and some houses in Mir Ali area and caused civilian casualties. They said dozens of civilians stuck up in the exchange of fire were also injured. They had been taken to hospitals in the region.