The UN regional coordinator for South Sudan, David Gressley, told journalists that the military buildup near the town of Abyei in central Sudan was jeopardizing a shaky 2005 peace agreement, the BBC reports.
"There's a gradual escalation of forces on all sides at this point in time," Mr Gressley said.
Mr Gressley said he did not think either side wanted a war, at this point, but that the situation had to be de-escalated or it could unravel the entire peace process.
Fighting flared in mid-May between northern and southern forces in Abyei, which sits on disputed, oil-rich land. The 2005 peace agreement called for joint north-south patrols of the town but did not resolve the town's status. The Voice of America reported that the escalation in central Sudan is high on the agenda of a delegation from the UN Security Council that is now visiting the country.
The fighting, which began on May 13, resulted in widespread destruction in Abyei and the displacement of as many as 50,000 people.
[Salva Kiir, the regional president of southern Sudan] confirmed reports this week that the Khartoum government of President Omar al-Bashir is deploying more than 1,000 additional troops to the disputed region.
"I have already called upon him [President Bashir] to intervene to order his military commanders to pull out their forces from Abyei area," he said. "We are not going to fight them."
The Security Council delegation is also expected to press Sudan on ending a separate conflict in the western region of Darfur that is threatening to spill over into a Chad-Sudan border war.
The delegation arrived early Thursday in Darfur, according to the Associated Press. The UN passed a resolution almost a year ago to create an international peacekeeping force in Darfur, but Sudan's government has only allowed about one-third of the force to deploy. The delegation plans to press Sudan on allowing in the remaining force as one priority.
At the top of the list was speeding up deployment of the United Nations-Africa Union force that took over peacekeeping in January but has only gotten 9,000 of the 26,000 authorized troops on the ground, a key to helping protect civilians in the many camps for the displaced.
One stumbling block has been the Sudanese government's reluctance to allow non-African troops into the region - and on this issue the council got a piece of good news Wednesday.
Britain's U.N. Ambassador John Sawers, co-leader of the council delegation, said [Nafie Alie Nafie, a Sudanese presidential adviser], promised that Thai and Nepalese battalions could deploy after Ethiopian and Egyptian troops arrive in Darfur.
On Tuesday, Richard Williamson, the US envoy in Sudan, postponed talks on normalising Washington's relationship with Khartoum, after years of sanctions, saying neither side was serious about maintaining peace.
"I won't be part of a sham peace that won't change the situation," he told reporters, referring to both the north-south tensions and the ongoing but separate conflict in Sudan's western Darfur provinces.
More than two decades of civil war in Sudan have left an estimated 2 million people dead and 4 million displaced, according to the International Crisis Group. The major divide is between the mostly Muslim, Arab north – which dominates the government – and the mostly Christian and animist south. Coveted oil resources in southern Sudan have raised the stakes in the conflict.
In 2003, a separate conflict intensified in the western Darfur region, where government-backed Arab janjaweed militias have attacked ethnic African civilians. That conflict has left some 300,000 dead, according to the United Nations. It's also turned into what many now see as a proxy war between Sudan and neighboring Chad to its west.
Meanwhile, The Washington Post reports that the International Criminal Court's top prosecutor implicated top Sudanese officials in recent atrocities in Darfur, including rape and killings. Chief prosecutor Luis Moreno-Ocampo was expected to deliver a report to the UN Security Council Thursday and present names and evidence to the court in July.
"Women and girls are raped. Victims are as young as five or six years old. Parents are forced to watch," he said in his report. "This is not an incidental byproduct of war. It is a calculated crime, intended to do irreparable damage to communities."
The Christian Science Monitor reported last month that Sudan's ongoing turmoil was destabilizing the region. Tensions between Sudan's government and neighboring Chad mounted after a Chad-backed Darfur rebel group carried out a daring raid, the first time violence had spread near the capital. The attack, carried out by the Justice and Equality (JEM) rebel group, left more than 200 dead.
The move by the Justice and Equality (JEM) rebel group … gives Khartoum a reason to ramp up its latest offensives in Darfur and raises the prospect of a border war between Chad and Sudan; both believe the other is using rebels as proxy fighters.