NAIROBI The armed wing of the National Council for the Defence of Democracy (CNDD), the main rebel movement in Burundi, Friday rejected an agreement for a civil war ceasefire by July 20 as totally unrealistic. The agreement was signed in the northern Tanzanian town of Arusha on June 21 by 17 Burundian delegations, including the CNDD and a team representing the regime of strongman Pierre Buyoya. But the Hutu guerrillas in the Forces for the Defence of Democracy (FDD) have split with the political leadership of the CNDD, and were at Arusha as an unofficial delegation without the right to sign the agreement.