JUBA (Reuters) - A South Sudanese rebel group on Friday accused government troops of attacking their base only a day after the parties signed a ceasefire in a four-year war that has killed tens of thousands of people.  The ceasefire, that would allow humanitarian groups access to civilians caught in the fighting, formally comes into force on Sunday morning.  On Friday afternoon, a spokesman for the SPLA-IO rebel group said army forces had attacked a rebel base in Deim Jalab, in the western part of the country. Lam Paul Gabriel said two rebels and five government troops were killed in the fighting. The army spokesman in the capital, Juba, did not immediately respond to a request for comment. The war that began in late 2013 in the world s youngest nation has forced a third of the population to flee their homes. The United Nations describes the violence as ethnic cleansing.  Earlier this year, pockets of the country plunged briefly into famine. The latest round of talks in the Ethiopian capital,  convened by the East African bloc IGAD, brought the warring sides back to the negotiating table after a 2015 peace deal collapsed last year during heavy fighting in Juba. After the new agreement was signed on Thursday, 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.  The German foreign ministry welcomed the agreement as an important step toward bringing peace to South Sudan.  We call on all participating parties to implement the agreement in a comprehensive and sustainable manner, and to ensure that humanitarian organizations are not hindered in doing their work,  a ministry spokeswoman said. 