JUBA - South Sudan should lift restrictions on transporting aid supplies using the River Nile, a vital mode of movement in the war-torn country that has almost no proper roads, the United Nations' humanitarian chief said on Saturday.South Sudan, the world's newest nation and one of its poorest, has been mired in a conflict since December 2013 that pits President Salva Kiir's SPLA army against troops loyal to rebel leader Riek Machar, a former deputy president.Fighting has killed thousands of people and displaced more than 2.2 million, 500,000 of whom have fled the country, since the country plunged into civil war. Many rely on aid to survive.The country relies heavily on the River Nile for transport due to a lack of tarmacked roads. The river was a lifeline for aid recipients until the government put restrictions on its use.In early June, government spokesman Michael Makuei Lueth said for security reasons all UN humanitarian cargo would be searched along highways, on water and at the Juba International Airport.UN Emergency Relief Coordinator Stephen O'Brien urged Kiir to "ensure the restriction on barges transporting vital lifesaving supplies on River Nile be lifted without delay," making the comments at a news conference in Juba.O'Brien visited Bentiu and Leer in Unity State County, which has experienced some of the worst fighting over its oil fields. On the trip he met relatives of victims of the conflict, some of whom he said had been raped.In July - in response to several complaints from UN humanitarian agencies - President Kiir directed that security personnel and other government agencies stop harassing humanitarian aid workers and allow for safe passage for them and their supplies."I call on all parties to uphold their obligations to comply with the principles of international humanitarian law and ensure the safe passage of humanitarian assistance," O'Brien said, adding 27 aid workers had been killed and others were unaccounted for during the conflict."We need rapid and unhindered access to provide assistance to conflict affected communities by the most efficient means possible - be that air, road or river."