Tension is high in Sudan as judges in The Hague have begun their ruling on a disputed internal border which cuts through rich oil fields.
The main parties in north and south Sudan have pledged to abide by the court ruling but some fear a return to their long war, which ended in 2005.
Under the peace deal, the south is autonomous but the region’s borders are not clearly defined.
Both north and south claim the Abyei region and its oil.
The Permanent Court of Arbitration will not decide on who owns the land, but will decide on where Abyei’s borders lie.
UN peacekeepers have beefed up their presence in Abyei in the run-up to the decision.
At the weekend the UN accused the southern Sudanese army, the SPLA, of moving troops into Abyei ahead of the ruling – claims the SPLA strongly denied.
Armies from the north and south had agreed to stay out of the area.
A 22-year civil war – separate from the Darfur conflict – which pitted the mainly Muslim north against the Christian and animist south ended in 2005, after claiming 1.5 million lives.
Under the peace deal a commission was established which demarcated Abyei’s boundaries – but the north rejected the commission’s decision.
Abyei was supposed to be administered jointly until a referendum in 2011, when residents will vote on whether to join the north or the south.
Analysts say Abyei’s residents are likely to vote to join southern Sudan’s administration, so the disputed area’s size and make-up is crucial.
At the same time, the south will vote on whether it wants to be independent.
The court in The Hague will decide if the boundary, which the commission ruled lies around 90km (56 miles) north of Abyei town, is correct – or it will demarcate a new border.
After clashes broke out in Abyei town last year, the two sides agreed to refer the case to The Hague.
As many as 100 people died, and the incident was seen as the biggest threat to the peace deal.
Correspondents say Abyei town is a ghost of its former self with few prepared to rebuild it, fearing further clashes.
Only about 3,000 people are thought to live in and around Abyei compared with 50,000 before the fighting last year, Reuters news agency reports.
The BBC’s James Copnall in the capital, Khartoum, says the area is home to an Arab group of cattle herders, known as the Misseriya – loyal to the north, and the Dinka Ngok, part of the largest ethnic group of the south.
There is competition for resources like land for grazing and water and the divisions can easily be exploited, analysts say.
Both sides were used as proxy armies during the civil war.