The leftist Sandinista government and rebel negotiators are close to agreeing on a definitive end to hostilities, sources on both sides say.

But a major stumbling block in the fourth round of the talks on achieving a lasting peace that begin in Managua Tuesday is a timetable for disarming the U.S.-backed rebels, the sources said Monday.The Sandinistas want the rebels, known as Contras, to disarm by September. The rebels want to do it by the end of January.

At the last parley May 26-28, the Sandinistas made concessions on what the rebels call the democratization of Nicaragua and pledged to enact political reforms the Contras demand.