The United Nations Security Council has demanded an “immediate withdrawal” by northern Sudanese armed forces from the disputed town of Abyei.

Sudan's northern army has seized control of the disputed, oil-producing Abyei region, officials said on Sunday. This has forced thousands to flee the area and has brought the country's north and south to conflict.

 The Sudanese army’s military operations “threaten to undermine the mutual commitment of the parties to avoid a return to war,” according to a statement by the UN at a news conference yesterday on Sunday in Khartoum. While falling short of a presidential statement, “it is very rare for the Security Council to express itself this way out of New York.”

The capture of Abyei, which took place hours after the arrival of a dozen top UN envoys in Khartoum, sparked concern violence may escalate between the two sides, which fought a two- decade civil war that ended with a U.S.-brokered peace agreement in 2005.

The Jan 9, referendum vote split the oil-rich south from the north, while both sides stake a claim to Abyei, which lies on the border between the two regions.

The South voted to declare them independent from the north in a January referendum, promised in the 2005 peace deal that ended the last north-south civil war.

That conflict between the Muslim dominated north and the south, where most people follow Christian and traditional beliefs, killed an estimated 2 million people.

“Escalatory military operations” by both sides as the clashes enter a fourth day were “serious” violations, the Security Council said.

A small group of southern army troops in Abyei have fought four battles and were overpowered by the Sudanese army, Philip Aguer, Southern Sudanese army spokesman, said on Sunday in Juba, the regional capital of Southern Sudan. The remaining troops withdrew from Abyei on May 21, he said.

The Southern Sudanese army rejected the accusations, and demanded an investigation into the attack.

“It is the duty of the Security Council to see that the Sudan Armed Forces are out of Abyei,” said Barnaba Marial Benjamin, Southern Sudan’s minister of information.

Abyei is contested between the region’s Ngok Dinka people, who are settled in the area and consider themselves southerners, and Misseriya nomads who herd their cattle south in the dry season and are supported by the Khartoum government.

The region postponed voting to decide whether to join the south or remain a special administrative region in the north since there are disagreements on who is eligible to vote.

Abyei produces less than 2,500 barrels of oil a day, according to Sudan’s Oil Ministry.