North Sudan forces have seized the disputed border town of Abyei, a move described by South Sudan as illegal and a "declaration of war."
The North occupied the strategic area late Saturday with 5,000 troops and tanks, the BBC reports. Some 20,000 people — almost the entire population of Abyei — have fled the fighting and are headed south, Doctors Without Borders said. The group said it has helped 42 people wounded in the violence.
Tensions in the oil-producing Abyei region are rising and there are fears that the latest clashes could spark a new North-South conflict, as South Sudan prepares for independence on July 9. Abyei's status remains to be determined, and the area is claimed by both sides.
After weeks of skirmishes between northern and southern armies, Khartoum sent 15 tanks into Abyei town on Saturday, a UN official said. The UN said its base had been hit by mortars.
"Most residents in Abyei town left yesterday (Saturday) and have gone south. There are reports of looting by armed groups," UN spokeswoman Hua Jiang told Reuters.
The UN Security Council has cancelled a visit to Abyei scheduled for Monday. The visiting delegation is planning to raise the Abyei issue during meetings with officials in Khartoum.
Abyei, a fertile area near several important oil fields, had been governed by a joint body comprising northerners and southerners. The country’s north is mainly Muslim, while in the south most people follow Christian and animist beliefs.
In a January referendum, southerners voted overwhelmingly to secede and become the world's newest nation. More than 2 million people were killed in decades of civil war between north and south. But control over Abyei remains a flashpoint as the secession of South Sudan approaches.
Southern military spokesman Col Philip Aguer said the North had committed an aggression, and called for the UN to intervene.
"We didn't declare war. The [Sudanese ruling] National Congress party and the Sudan armed forces declared war on us," Aguer told the Associated Press.
"If the international community do not intervene quickly to rescue the situation then this is a complete violation of the comprehensive peace agreement, a complete violation of the ceasefire, and it is a declaration of war by Khartoum," he told the BBC.
The northern army said it acted after 22 of its personnel were killed in an ambush by southern armed groups last week. North Sudan said it had sent in the troops to clear out southern soldiers that it said had entered the area in violation of earlier agreements.
"The Sudanese armed forces control Abyei and are cleansing it of illegal forces," Amin Hassan Omar, a minister of state for presidential affairs, told reporters in Khartoum after meeting a UN Security Council delegation.
The southern army accused the north of shelling villages and said it had withdrawn its forces from the town of Abyei after the north moved in.
Both the United States and Britain have condemned the escalation of violence in the Abyei region. The White House said it was "disproportionate and irresponsible," and urged northern and southern leaders to meet and negotiate a settlement.