NAIRIBI: The US has ordered all non-emergency staff in South Sudan to leave, the State Department said on Sunday, as rising tensions provoke international concern. A fragile power-sharing agreement between President Salva Kiir and First Vice President Riek Machar has been threatened by recent clashes between their allied forces in the northeastern Upper Nile State. On Friday, a UN helicopter came under attack during a rescue mission, which killed a crew member.  An army general also died during the operation, the UN said.