ARSHAD MOHAMMED and JULIO -CESAR CHAVEZ A prominent South Sudanese economist fled to the United States on Thursday and accused South Sudan's President Salva Kiir of ordering a hit squad to kill or kidnap him in Kenya, an allegation immediately denied by the Juba government. Peter Biar Ajak landed at Dulles Airport outside Washington with his wife and three small children after travelling from Nairobi. He told Reuters that top South Sudanese officials whom he declined to identify had warned him that Kiir had ordered a team to abduct or murder him in the Kenyan capital. Ajak said SUVs with…