Heavy fighting involving tanks and helicopters raged in South Sudan yesterday between troops loyal to the president and those backing the vice-president, risking a return to civil war and further instability in a turbulent and poor region.Clashes between the forces of President Salva Kiir and Riek Machar - a former rebel leader who became vice-president under a deal that ended a two-year civil war - have killed hundreds of people since they broke out in the capital Juba four days ago.The violence intensified early yesterday. A witness saw two helicopters firing in the direction of Machar's headquarters. Residents reported tanks on the street. An official said heavy gunfire had erupted around UN bases, where hundreds of civilians had fled to shelter.The fighting broke out last week as South Sudan prepared to mark five years of independence from Sudan at the weekend.The UN Security Council demanded on Sunday that Kiir and Machar rein in their forces.The violence has raised fears of a return to the civil war that erupted in late 2013 and broadly ran along ethnic lines, pitting Kiir, an ethnic Dinka, against Machar, a Nuer.The conflict killed thousands of people, forced more than 2.5 million from their homes and left almost half the population of 11 million struggling to find food. Oil production, the biggest source of government revenue, plummeted.A new flare-up risks driving yet more people to refugee camps in neighbouring nations and further destabilising the region.Central African Republic is riven by conflict, the eastern region of the Democratic Republic of Congo is contending with a patchwork of militia and rebel groups and Burundi is embroiled in a violent political crisis.Machar blamed Kiir for a heavy helicopter bombardment yesterday."This tells [us] that our partner is not interested in peace," he said on Twitter.There was no immediate comment from Kiir's side.There has been no official death toll but at least five soldiers died on Thursday and a health ministry source said 272 people, including 33 civilians, were killed on Friday.Kiir and Machar ordered a ceasefire late yesterday, raising hopes of an end to the deadly violence. ..