Addis Ababa - US President Barack Obama will on Monday try to build African support for tough action against South Sudan's warring leaders if they reject an August peace ultimatum, US officials said.Obama will meet with leaders from Ethiopia, Kenya and Uganda as well as Sudanese Foreign Minister Ibrahim Ghandour in Addis Ababa to try and build a collective front to end the 19-month-old civil war in the world's youngest nation.Signalling a deeper commitment to ending violence that has killed tens of thousands of people and forced more than two million from their homes, Obama is expected to make the case for tougher sanctions and a possible arms embargo.South Sudan's rival leaders - who will not be at the meeting - effectively face an ultimatum, a "final best offer", according to one senior administration official."I don't think anybody should have high expectations that this is going to yield a breakthrough," the official said."The parties have shown themselves to be utterly indifferent to their country and their people, and that is a hard thing to rectify."Multiple agreements and ceasefires have failed, leaving violence and human rights atrocities unchecked.The latest agreement would sketch out rules for governance of the economy and the security sector and institute power-sharing between President Salva Kiir and his rival and former vice president Riek Machar, who now leads a rebel army.It could also establish a body to investigate possible genocide and war crimes.But with the outcome in doubt, the United States seems ready to move to isolate Kiir and Machar and backing a new transitional government.Speaking in Kenya on Saturday, Obama described the situation in South Sudan as "dire" as he called on leaders to "put their country first" - alluding to growing international sentiment that Kiir and Machar's personal rivalry is at the heart of the widespread carnage.Reluctant custodian South Sudan, midwifed into existence by US cash and support in 2011, has faltered badly in its infancy, and the Obama administration has been accused of abandoning the fragile nation."America bears a unique responsibility to end the war in South Sudan given its role in the creation of the country," said Casie Copeland of the International Crisis Group."The failure to more seriously try and bring about peace is an abdication of its moral responsibility."Obama has been urged to use his political weight to solve the crisis, with advocates saying it offers him the prospect of a lasting presidential legacy in Africa.He is wildly popular in South Sudan, as in other parts of East Africa.But White House aides are said to be reluctant, seeing little chance of success while the United States faces more immediate national security threats.At minimum, rights groups want to see the Obama administration take a tougher stance."President Obama should use his engagement on the issue to make clear that there will be accountability for economic crimes, including pillage and grand corruption," said Akshaya Kumar of the Enough Project.The White House has also been criticised for not pressing regional allies hard enough, particularly Uganda, whose troops are fighting alongside Kiir's army and against the rebels. Sudan is accused of aiding the rebels.Both countries play a part in the troubled East African effort to resolve the crisis, led by the regional eight-nation IGAD bloc, and will be the focus of attention on Monday.The IGAD initiative - which includes many of the parties scheduled to be at Monday's meeting - is widely seen as a failure.The IGAD peace process has seen scores of South Sudanese delegates hosted in luxury hotels in Addis Ababa at a cost of at least $20m, and each peace agreement quickly falling apart - sometimes within hours.