NAIROBI, Dec 31 (Reuters) - The South Sudanese government and rebels loyal to former Vice President Riek Machar have agreed on a ceasefire as they prepare for talks to end violence in the oil producing country, the IGAD group of East African countries said on Tuesday.

"President Salva Kiir Mayardit and Dr. Riek Machar agree on a cessation of hostilities and appoint negotiators to develop a monitored and implemented ceasefire," the bloc statement said.

There were no details on when exactly the ceasefire would take effect. IGAD has been mediating between the two sides. (Reporting by Drazen Jorgic; editing by Richard Lough and Patrick Graham)