A Congolese military helicopter flies over Rwandan troops preparing to march across the border at Goma in eastern Congo, February 25, 2009. A U.N.-backed Congolese military operation to oust extremist Hutu rebels from eastern Congo has caused more civilian casualties than damage to rebels, with more than 1,400 people deliberately killed over a nine-month period from January to September, according to Human Rights Watch and Oxfam. Congolese soldiers being fed and supplied with ammunition by the United Nations have killed civilians, gang-raped girls and cut the heads off some young men they accuse of being rebels or supporting the enemy, the human rights groups said. Rwandan Hutu rebels melted into eastern Congo's bush before advancing Rwandan and Congolese forces, and the surprise joint offensive did not quash militia groups at the heart of 15 years of conflict.