Sudan is gearing up for a January referendum that is part of a 2005 peace agreement that ended a simmering civil war in the troubled African nation.
Ibrahim Gambari, the U.N. special envoy to Sudan, said from Khartoum that he was worried conflict may erupt between rebel and pro-government forces.
"I call upon all parties to refrain from further offensive military action and respect international humanitarian law," he said in a statement.
Voter registration started Monday and runs through Dec. 1 in Sudan and eight other countries. U.N. authorities described the registration process as a "critical phase" in the 2005 peace agreement.
"We hope that all Southern Sudanese, wherever they live, will be able to turn out in peace and register for the referendum. We know that organizing voter registration has not been easy, given the country's size and the scale of the process, but we remain confident that it can be completed successfully," said Benjamin Mkapa, a Tanzanian diplomat and head of a three-member U.N. panel on Sudan, in his statement.
Voters Jan. 9 will decide of South Sudan breaks away from the rest of the country. A separate measure on the same days gives residents in the central area of Abyei the right to decide if they want to join the north or the south.