The U.N. High Commissioner for Refugees (UNHCR) estimated that 200 to 400 civilians were killed in two attacks in southeastern Chad late last month, a figure significantly higher than previously thought, U.N. deputy spokeswoman Marie Okabe said Tuesday. The UNCHR team was unable to provide exact numbers of the dead because most were buried in mass graves where they were killed. Those who were not killed in the initial attack died subsequently of dehydration and exhaustion, Okabe told reporters. On Sunday, an inter-agency assessment mission led by UNHCR to Tiero and Marena found a situation that one of our staff described as apocalyptic , UNHCR said. The March 31 attack on the two villages caused more than 18,000 people to flee their homes in the surrounding area. The majority of the internally displaced population is comprised of women and children, UNHCR said. The whereabouts of many men remain unknown. The UNHCR team said that a survey of the area indicated that people had little or no time to flee. Along the route could be seen belongings abandoned along the way by those who collapsed and were brought to Goz Amir camp health center for treatment, or who died where they fell, the refugee agency said.