Juba - UN aid chief Stephen O'Brien warned on Thursday of the "devastating" toll on the people of South Sudan of 19 months of a civil war marked by atrocities.O'Brien is on a four-day visit to South Sudan to see "first-hand the humanitarian consequences of the conflict, and efforts by aid organizations to respond to escalating needs," according to a UN statement."The food security situation is alarming," the statement added. "Nearly 70% of the country's population - 7.9 million out of 11.6 million people - are expected to face food insecurity this rainy season".O'Brien, the UN Under Secretary General for Humanitarian Affairs, met with some of the 166 000 civilians crammed into UN peacekeeping bases for fear of attack, in many cases since the war began in December 2013."From speaking to communities displaced in Juba, it's clear the brutal war has taken a devastating toll," he said, after touring the camp in Juba where aid workers are struggling to stamp out a cholera outbreak that has killed at least 39 people.South Sudan's civil war began in December 2013 when President Salva Kiir accused his former deputy Riek Machar of plotting a coup, setting off a cycle of retaliatory killings that has split the poverty-stricken, landlocked country along ethnic lines.Tens of thousands have died in the war, according to the UN, but no official death toll has been kept.Fresh peace pushPeace talks, led by the eight-nation East African IGAD bloc, have been going on in Ethiopia almost as long as the war, resulting in at least seven failed agreements and ceasefires, all broken within days or even hours.Mediators on Thursday gave them an August 17 deadline to sign a deal, the latest in a string of ultimatums issued since the conflict started.A new proposal to set up an "inclusive transitional government" was adopted on Thursday by mediators, even though many of the points on power sharing had been previously rejected by both sides."This is a comprehensive and final agreement that addresses all the problems of South Sudan that led to this crisis: the governance, the management of the economy, the security sector and the power sharing," chief mediator Seyoum Mesfin said.The war has forced 2.2 million people to flee their homes, with over 600 000 of those now refugees in the neighbouring nations of Ethiopia, Kenya, Sudan and Uganda. O'Brien is due to visit the UN base in Bentiu, capital of the northern battleground state of Unity, where over 100 000 civilians are sheltering inside a camp, more than live in the destroyed town itself.'Inhumane conditions'Both the government and rebels are accused of carrying out atrocities, including gang rape, burning people alive and the recruitment of large numbers of child soldiers.Aid agencies including Doctors Without Borders (MSF) have warned of restricted access to some of hardest hit areas, including to the northeastern battleground state of Upper Nile.The army denied on Thursday it had blockaded boats travelling on the Nile River - the main route for aid into the northern areas, including large areas under rebel control - but said ships were coming under attack."Rebels are using motor boats to attack...so for the sake of security they are advised to wait," army spokesperson Philip Aguer said. "There is no closure of the river transport, but it is advice."But aid workers said they were struggling to get key supplies in."MSF is deeply concerned about the continued denial of access for aid organisations to conflict areas and other remote areas," it said in a statement."People are being exposed to continual violence, increased displacement, fear of attacks, disease outbreaks and the risk of starvation," MSF added, warning of "inhumane conditions".