WASHINGTON (Reuters) - African countries should do more to pile pressure on South Sudan s political leaders, who seem incapable of resolving the country s four-year civil war, a top U.S. diplomat said on Wednesday.  We think there is more our African colleagues can and should be doing at this point, especially in terms of focusing on leadership, that from our point of view is behaving in a way that is very irresponsible,  Tom Shannon, U.S. under-secretary for political affairs at the State Department, told reporters. Shannon, speaking on the sidelines of a U.S-African Partnerships event at the U.S. Institute of Peace, said Washington had grown  intolerant  of South Sudan s leaders and the challenge was to work with African countries which are interested in seeing an end to the conflict.  This is a manmade conflict of horrific dimensions, which is about political leaders measuring each other through force at the cost of their populations,  said Shannon. His remarks come after the new USAID administrator, Mark Green, traveled to Juba on Sept. 2 to meet with South Sudan s President Salva Kiir and deliver a message from Washington that the United States was reviewing its relationship with his government.  The Trump administration last week imposed sanctions on two senior South Sudanese officials and the former army chief for their role in the conflict, atrocities against civilians and  attacks against international missions in South Sudan.  South Sudan became the world s newest nation when it gained independence from Sudan in 2011. War broke out in late 2013 and more than a quarter of its population of 12 million have fled their homes.  A confidential report by the United Nations last week said competing efforts to end South Sudan s civil war were allowing Kiir s government to exploit divisions among the international brokers. East African leaders said in June they want the warring sides to recommit to a peace deal they abandoned more than a year ago. . Among the international bodies trying to end the conflict are regional block IGAD, the U.N. Security Council, a troika of South Sudan s main Western backers prior to independence and an African Union panel. 