BAGHDAD, April 17 (Reuters) - Insurgents blew up bridges over a main highway from Baghdad to southern Iraq, further disrupting the country's battered transport system, witnesses said on Saturday. They said debris from blasts targeting at least two bridges closed the Baghdad-Hilla road, one of two highways leading to the port city of Basra. The U.S.-led administration said in a statement that two highways, including the Basra-Hilla route, were closed for repairs and to help halt guerrilla attacks against U.S. forces and supply convoys. The U.S. military said it had no information about blasts targeting highway bridges. But a traveller...