The UN's peacekeeping chief has warned an increase in troops along the border between north and south Sudan would not prevent or even contain clashes.
Alain Le Roy was addressing the UN Security Council as it considered reinforcing its mission on the boundary in the run-up to January's referendum.
The south is to decide whether to secede as promised in the 2005 peace deal to end two decades of war.
Tension in Abyei, along the border, was of critical concern, Mr Le Roy said.
The people of Abyei are also due to vote in a separate referendum in January on whether the oil-rich region should belong to north or south Sudan.
There have been fears that delays in either vote could lead to violence.
'Time running out'
Both sides have promised that the January referendum on southern self-determination will be held on time and its results respected.
But Mr Le Roy said preparations were way behind schedule and tensions were rising, with the two armies accusing each other of building up forces along the border.
The UN had not been able to fully verify the situation, but there did not seem to be a major military mobilisation, he said.
The UN is looking at ways to increase its presence in areas that could be flashpoints for conflict.
One option was to redeploy soldiers from other parts of Sudan; another was to request additional troops, Mr Le Roy said.
But he said a political agreement on outstanding issues, especially in Abyei over border demarcation, voting rights and control of oil-wealth, was the best tool to prevent a return to civil war.
"Time is running out for resolution of the many outstanding issues," he said.
Sudan's UN ambassador Daffa Alla Elhag Ali Osman also said it was crucial that these issues be resolved, otherwise there was a chance that war could erupt.
The UN-mediated talks on Abyei's referendum, due to resume at the end of the month, have been postponed.
Former South African President Thabo Mbeki, an African Union negotiator, said more consultation was needed before the meeting.
At least 100 people died and 50,000 people fled their homes in heaving fighting in Abyei two years ago.
The UN has about 10,000 personnel in the south of Sudan.
A 22,000-strong joint United Nations and African Union peace force is on the ground in the western region of Darfur dealing a separate conflict.