By James Copnall
BBC News, Khartoum
Much of Abyei was left in ashes after clashes last year
The South Sudanese army, the SPLA, has denied breaking a peace deal after the UN accused its troops of moving into the contested region of Abyei.
Abyei is coveted by both the north and the semi-autonomous South, in part because of its oil reserves.
Both armies had agreed to stay out of the area, whose boundary is to be decided by international arbitration.
Abyei has been at the heart of a long-standing dispute that threatens a 2005 peace deal which ended years of war.
The UN head in Sudan had accused SPLA soldiers and policemen of what he called a "clear violation" of a deal to ensure peace in the region.
Ashraf Qazi said the presence of the armed men, principally in the town of Agok, could lead to escalation and violence.
But the spokesman for the SPLA - Sudan People's Liberation Army - categorically denied that his troops have moved illegally into Abyei.
Maj Gen Kuol Diem Kuol told the BBC he had checked with his commanders and said they were stationed in their normal places.
He rejected reports there was a troop build-up near the border.
On Wednesday, the Permanent Court of Arbitration in the The Hague will give a final ruling on the boundaries of Abyei.
When a local ruling was made last year, clashes broke out between soldiers from the north and the South.
As many as 100 people died, and the incident was seen as the biggest threat to the 2005 peace deal signed by the two sides.
Last Thursday, both reiterated their commitment to respect the Court of Arbitration ruling and said they would work to minimise local tensions.
But the verdict on Abyei is seen as a key test of the peace process.
In 2011 Abyei will vote on whether it wants to join the north or the South, while the South will vote on whether it wants to be independent.