New York - The United States warned on Thursday that a new regional plan to shore up South Sudan's failing peace agreement was the last chance for Juba's leaders to end the war, now in its fourth year.US Deputy Ambassador Michele Sison told the UN Security Council that Washington will reconsider its support for the deal signed two years ago by President Salva Kiir and rival Riek Machar, if the latest effort fails.The remarks highlighted growing impatience by the US administration with leaders in South Sudan, which won independence from Sudan just six years ago with strong backing from the United States."It is high time for action," Sison told the council.The plan agreed by the regional IGAD group is "the last chance for salvaging the peace agreement," she said."If South Sudan's leaders do not participate in this high-level forum in good faith, and stick to its deadlines, the United States will need to review our position and priorities on support for the peace agreement and its implementing bodies."The seven-nation IGAD group agreed this month to launch the so-called "revitalization forum" to push for a permanent ceasefire and talks on ending the war, the latest bid by the regional leaders.Ethiopia's Ambassador Tekeda Alemu said the region was now speaking with "one voice" on South Sudan and that there was "no viable alternative" to the regional plan to support a settlement.The United States has repeatedly called for an arms embargo and sanctions on those who are blocking peace efforts in South Sudan. A US bid to impose the tough measures failed to win support at the council in December after Russia, China, Japan, Egypt and four other countries abstained in a vote on a US-drafted sanctions resolutions."The Security Council waits and waits," Sison deplored. "We wait for things to change in South Sudan, but nothing is changing in South Sudan.""The violence continues. Horrific atrocities are reported so often that they become almost routine."Tens of thousands of people have been killed and more than 3.5 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 and 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.