KHARTOUM/JUBA Tens of thousands of people fled Sudan's contested Abyei region as northern militias accused of helping seize the area over the weekend moved further south, the United Nations said on Wednesday.
Armed groups, thought to be northern militias, also opened fire on four U.N. helicopters in Abyei on Tuesday, a U.N. spokeswoman said.
North Sudan sent tanks into Abyei, a central, oil-producing region claimed by both north and south Sudan, on Saturday, sparking an international outcry.
The move came at a highly sensitive time for Sudan, less than seven weeks before the country's south is expected to declare independence from the north, as part of a 2005 peace deal that ended decades of civil war.
Abyei was a key battleground in Sudan's last civil war and a symbolic emblem for both sides. The region is used all year round by the south-linked Dinka Ngok people and for part of the year by northern Arab Misseriya nomads.
Analysts fear further north-south fighting over the region could spark a return to full-blown conflict, a development that could have a devastating impact on the surrounding region.
U.N. Secretary-General Ban Ki-moon urged north and south to return to the negotiating table and resist "further acts of aggression."
"We must all impress on the parties that military confrontation in Abyei is not an option," he told heads of state at an African Union meeting in Addis Ababa.
Thousands of Abyei residents initially fled to the town of Agok, just over the border into south Sudan, said aid groups.
U.N. and aid agencies said up to 40,000 had now been forced to leave their homes and were moving deeper into south Sudan.
"There are enormous numbers of people now on the road from Agok to Turalei, on muddy roads. Many kids need to be treated for dehydration," said Gustavo Fernandez, program manager with aid group Medecins Sans Frontieres.
Most refugees in Agok were living under trees, while 2,800 had found shelter in a local school, said the Anglican Alliance aid group.
"Civilians are down on streets and in bushes, no food, no shelter, no water and no medical assistance," the Anglican church umbrella group said in a report.
Misseriya militias started pushing further south after people left the region's main settlement Abyei town, said U.N. spokeswoman Hua Jiang.
"There are reports that they are moving south," she said.
Southern army spokesman Philip Aguer accused the north of using the Misseriya to carry out a land-grab ahead of the separation of the south.
"Misseriya are being transported by SAF (the northern army) to Abyei. They want to claim the land," he told Reuters.
U.N. General Secretary Ban Ki-moon urged north and south to return to the negotiating table and resists "further acts of aggression."
"We must all impress on the parties that military confrontation in Abyei is not an option," he told heads of state's at an African Union meeting in Addis Ababa.
Sudan President Omar Hassan al-Bashir defied U.S., U.N. and other calls to pull back, saying Abyei belonged to the north. "We will not withdraw from it," he said in Khartoum on Tuesday.
The north's show of force could shake a fragile political balance that has held in Africa's largest country since the 2005 deal ended the civil war that left millions dead.
It could also delay the restoration of normal diplomatic relations between north Sudan and the outside world.
The United States on Monday ruled out dropping it from a terrorism list and restoring Washington's ambassador to Khartoum if it continues to occupy Abyei.
A total of 14 rounds were fired when the United Nations helicopters took off from a U.N. compound in Abyei town on Tuesday but they landed safely, U.N. spokeswoman Jiang said.
Jiang said Misseriya militias supported by Khartoum were probably responsible for the attack.
Fighting and looting -- some of which targeted supply bases of U.N. agencies -- had died down, she added.
Northern forces had deployed military aircraft at a northern airbase in El Obeid within striking distance to Abyei, said the Satellite Sentinel Project (SSP), a monitoring group.
"The presence of these attack-capable planes in close proximity to Abyei is consistent with reports of SAF (northern) bombing attacks in Abyei within the past five days," said SSP.
No reliable casualty figure have emerged yet as aid agencies struggle to reach Abyei. Britain's ambassador in Khartoum, Nicholas Kay, wrote on his blog that two embassy staff had lost relatives in the disputed region.
(Additional reporting by Aaron Maasho in Addis Ababa; Writing by Tom Pfeiffer in Cairo; Editing by Matthew Jones)