ADDIS ABABA (Reuters) - South Sudan s government and rebel groups signed a ceasefire on Thursday in the latest attempt to end a four-year civil war and allow humanitarian groups access to civilians caught in the fighting. The ceasefire aims to revive a 2015 peace deal that collapsed last year after heavy fighting broke out in South Sudan s capital Juba. It was agreed after talks in the Ethiopian capital Addis Ababa convened by regional bloc IGAD. A decision by President Salva Kiir to sack his deputy Riek Machar triggered the war in the world s youngest country. The war has been fought largely along ethnic lines between forces loyal to Kiir, who is Dinka, and Machar, who is Nuer. Tens of thousands have died and a third of the population of 12 million have fled their homes. The conflict has since mutated from a two-way fight into one involving multiple parties and this has made it harder to find peace. Representatives of Kiir and Machar were both present at the signing. South Sudan s Information Minister Michael Makuei Leuth told journalists:  The cessation of hostilities will be effective 72 hours from now. As of now, we will send messages to all the commands in the field to abide by this cessation of hostilities.   From now onwards, there will be no more fighting,  he added.  Just talks.   I do hope in signing this agreement, you will try to put an end to this tragedy .... This is an encouraging first phase,  said Moussa Faki Mahamat, chairperson of the African Union Commission. Ethiopia s Foreign Minister Workneh Gebeyehu, also present, said:  There is no longer any excuse for the violations of human rights. All parties are obliged to observe cessation of hostilities agreement. Diplomats at the talks told Reuters the next phase of the negotiations would now center on thrashing out a revised power-sharing arrangement leading up to a new date for polls. The United States, Britain and Norway, which form a group that supported a 2005 accord leading to the independence of South Sudan from Sudan, welcomed the agreement.  The troika ... congratulate the parties on their willingness to compromise for the benefit of the people of South Sudan and hope that they immediately take action to make good on that agreement,  they said in a statement issued by the U.S. State Department. 