Almost 24 hours after a group of at least 20 Islamic State (IS) terrorists attacked a prison in Afghanistan's Jalalabad city, 21 people, including students, have been killed and 43 injured in a fierce gun-battle which is still ongoing.Sources in Kabul said that the 21 people, including three attackers, were killed in the gunfight between the terrorists and the Afghan security forces in the Jalalabad prison and a nearby mall.The prison was breached from multiple points with a combination of Suicide Vehicle Borne Improvised Explosive Devices (SVBIEDs), rocket attacks and ground attackers.The IS attackers are wearing suicide vests, the sources said, adding that civilian homes in the vicinity were damaged by grenade and mortars.The attackers also entered residences and executed some civilians.Sources said the IS's foreign terrorists are involved in the attack on the prison in which 700 prisoners attempted to escape. Most of the prisoners were recaptured, police sources said.However, at least 50 escaped, the sources said, adding that the prison break attempt may be a reaction to the killing of Assadullah Orakzai, Pakistani-origin terrorist who was the IS- Khorasan Head of Intelligence.He was killed by the National Directorate of Security (NDS), the Afghan intelligence agency on Saturday in an operation near Jalalabad.The counter-terror operations in Jalalabad on Monday are going slowly to prevent killings of civilians and minimize the collateral damage, sources said.Last November, the Afghan and NATO coalition forces defeated the ISKP, forcing hundreds to surrender along with their families, with most of them being Pakistani nationals who had infiltrated, four years ago, into Nangarhar province through the Durand Line -the border which Afghanistan has disputed since it was drawn by the British in 1893.Nangarhar province was the centre of the ISKP activities in Afghanistan.Jalalabad, the fifth-largest city of Afghanistan, is about 80 miles from the capital Kabul.