A female Tamil Tiger suicide bomber hiding among a group of civilians fleeing Sri Lanka's war zone blew herself up on Monday, killing at least 28 people and wounding 90, the military said. The blast happened near Vishvamadu, a town in the north of the Indian Ocean island recently captured by the military as it battles to crush the Liberation Tigers of Tamil Eelam (LTTE) separatists and end a 25-year-old war. Underscoring the challenge the military faces in separating combatants from civilians as the war moves to a final showdown, officials said the bomber was in a group of people being checked for weapons by soldiers when she set off the bomb. "Two officers and 18 others were killed from the army, two officers injured and 48 others injured. Eight civilians were killed and 40 others injured. Many of them are women and children," military spokesman Brigadier Udaya Nanayakkara said. The rate of civilians fleeing fighting between the military and the cornered LTTE has picked up sharply this week, with at least 17,300 fleeing since Thursday -- out of at least 21,627 so far this year, according to military tallies. More than 50,000 soldiers are converging on a sliver of jungle in the Indian Ocean island's northeast to crush the LTTE -- now estimated to number no more than 2,000 hardcore fighters. "We are trying to save Tamil people, but LTTE is not allowing us to do that and they have exploded a suicide bomb among civilians," Media Minister Anura Priyadarshana Yapa told Reuters. The government, aid agencies and rights groups have accused the LTTE of forcibly keeping civilians in the war zone as human shields and for use as conscripts and laborers. The rebels deny that, saying Tamils there do not want to leave their side. They accuse the government of intentionally targeting civilians, which the military denies.