The Stuart Highway has been closed in both directions following a serious crash in South Australia's arid far north.Police were called to the scene near the remote sheep station of Bulgunnia, 100 kilometres north of Glendambo, about 12:30am today, following reports of a collision between a road train and a vehicle.The condition of those involved is unknown.Major crash investigators have been sent to the scene.Senior Constable Jen Cullinan said it is unclear how long the road will remain closed.Police are asking motorists travelling north to stop at Glendambo, and those travelling south to stop at Coober Pedy.