Sudan's northern army vowed to hold territory it seized in the disputed oil-producing region of Abyei, defying a U.N. demand it withdraw and pushing the north and south closer to conflict as the south prepares to secede.

Khartoum sent tanks into Abyei town, the area's main settlement, on Saturday, the United Nations said after weeks of growing tension and accusations of skirmishes by both sides.

The U.N. Security Council has called on the north to immediately withdraw forces from the positions taken in Abyei. Representatives of council members were expected to meet the southern government in Juba on Monday after talks in Khartoum.

Thousands of people fled, leaving Abyei town empty, while food supplies have also been disrupted, an aid organisation said. U.N. personnel based in the area said they had not been able to resume patrols of the region due to fighting.

Analysts fear north-south fighting over Abyei could reignite civil war, a move that would plunge the nation back into chaos as the south splits away on July 9 and could send refugees back across the borders to neighbouring African states.

"Free citizens, your armed forces will hold all areas which the laws and agreements entrust to it. They will work on establishing peace and stability so the government can reach a solution and accord to ensure security and stability in the region," the northern army said late on Sunday.

The statement was carried by state television's website.

North Sudan says it sent in troops to clear out southern soldiers who it said had borken agreements by having entered the area.

Control over Abyei remains the biggest point of contention in the countdown to the secession. Abyei contains fertile grazing land and oil. It has symbolic importance for both sides.

RESIDENTS FLEE

The 2005 peace deal that ended two decades of civil war promised Abyei residents a referendum over whether to join the north or south but that never took place as neither side could agree on who was qualified to vote.

Fighting in Abyei forced thousands to flee their homes from Abyei town and surrounding villages to the southern Warrap state.

"Abyei town is now empty. We saw trucks loaded with refugees today," said Gustavo Fernandez, programme manager at charity Medecins Sans Frontieres (Doctors without Borders).

About 50 wounded civilians had been treated, while the escalation had disrupted food supplies in Abyei, he said.

"Most markets in Abyei are closed," he added.

A U.N. official said late on Sunday it was unclear when the U.N mission in Sudan could resume patrols in Abyei area.

Instability in Abyei could also spread to neighbouring northern state of South Kordofan where the southern ruling party accused Khartoum of rigging governorship elections earlier this month, analysts say.

The U.S. ambassador to the United Nations, Susan Rice, said the Sudanese foreign minister and vice president had cancelled a meeting with the council representatives in Khartoum. She said this was a lost opportunity to exchange views.

Southerners overwhelmingly voted to declare independence 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 mainly Muslim north and the south, where most people follow Christian and traditional beliefs, killed an estimated 2 million people.