ADDIS ABABA (Reuters) - The United States is considering how to pressure South Sudan s President Salva Kiir into peace, but withdrawing aid may not work, U.S. envoy to the United Nations Nikki Haley said ahead of a visit to South Sudanese refugees in Ethiopia on Tuesday.  Haley plans to visit Gambella in western Ethiopia, where nearly 350,000 refugees have flooded across the border from South Sudan since the country spiraled into civil war in 2013, just two years after it gained independence from Sudan.   You have to really think hard before you pull U.S. aid because President Kiir doesn t care,  Haley said.  He doesn t care if his people suffer and that s the concern we have as we don t know that will make a difference.   That s a conversation we will have and we will try and see exactly what will move President Kiir so that he does ... start to really look at creating a safe position for his people,  she told reporters in Addis Ababa late on Monday. Haley will this week be the most senior member of U.S. President Donald Trump s administration to visit South Sudan, where she is due to meet with Kiir. But first, she will see how the conflict has threatened to spillover through deadly cross-border raids into Gambella, Ethiopia by gunmen from South Sudan.  Trump s new aid administrator, Mark Green, visited South Sudan last month, telling Kiir that Washington was reviewing its policy toward his government. He called on Kiir to end the violence and implement a  real  ceasefire.  The war in South Sudan was sparked by a feud between Kiir and his former deputy Riek Machar. It has plunged parts of the world s youngest nation into famine and forced a third of the population - some 4 million people - to flee their homes. Machar fled and is now being held in South Africa to stop him stirring up trouble, sources told Reuters in December.  A fragile peace deal broke down last year amid gun battles between soldiers and rebels in the capital Juba. International efforts to bring warring sides to new talks have not succeeded.  We can t see any more deaths, we can t see anymore famine, we ve got to start seeing the situation start to get better and I think that the pressure is only going to continue until President Kiir makes a difference,  Haley said.  The Trump administration last month 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. 
U.S. mulls South Sudan pressure, cutting aid may not work: U.N. envoy