New York - US Ambassador Nikki Haley said on Tuesday she will tell South Sudan's leaders when she visits Juba next month to seize a final chance for peace and quickly revive a political deal to end the three-year war.President Donald Trump last week announced that Haley, his high-profile ambassador to the United Nations, would travel to South Sudan and to the region to push for an end to conflict.Addressing the UN Security Council, Haley said peace efforts in South Sudan must show progress in the coming weeks after a 2015 deal backed by the United Nations never took hold. "This is the last chance at salvaging the peace agreement in South Sudan," Haley said."The different parties to the conflict must use the next several weeks to commit themselves to this process and to conclude it," she added.There has been growing impatience with President Salva Kiir as the humanitarian crisis in the country worsens, with 7.6 million people now in need of aid.In a shift, the African Union last week warned that it will consider sanctions unless there is progress towards a political deal under a new peace drive led by the seven-nation IGAD regional group.The new initiative, backed by the Security Council, is expected to hold its "revitalization forum" for South Sudan in the coming weeks, in what is shaping up as a test for Juba's leadership.UN envoy for South Sudan David Shearer told the council that the "government appears emboldened by its recent military gains, including in some key strongholds of the SPLA in opposition loyal to Riek Machar."Tens of thousands of people have been killed and nearly 4 million people have been forced to flee their homes in the conflict that has stirred ethnic rivalries between Dinka, Nuer and other groups.The war began when Kiir accused Machar, his former deputy, of plotting a coup. Heavy fighting flared in Juba last year when Machar returned to the capital under the terms of the peace deal. Machar has since been residing in South Africa and regional countries, at the request of the United States, are preventing him from returning to South Sudan.