The UN has asked South Sudan to investigate separate attacks on UN peacekeepers in Abyei last week [EPA]

Four UN helicopters were shot at, probably by militias allied to northern Sudan forces, during a visit to the disputed Sudanese region of Abyei late on Tuesday, according to a UN spokeswoman.

A total of 14 rounds were fired when the helicopters took off, but the crews landed safely, Hua Jiang , a UN spokeswoman said on Wednesday.

Jiang said militias of the Arab Misseriya tribe supported by Khartoum were probably responsible for the attack, adding that they were now moving southwards after civilians had left the main settlement of Abyei.

"There are reports that they are moving south," she said.

Jiang said fighting and looting in Abyei had stopped after inhabitants left, adding that some stockpiles of UN agencies had been looted.

Sudan's northern army moved tanks into Abyei town, the border area's main settlement, on Saturday, sparking an international outcry and forcing thousands to flee.

On Tuesday, Omar Hassan al-Bashir, Sudan's president, refused to withdraw troops from Abyei, defying international calls for a withdrawal, and raising the stakes in a standoff with the south.

"Abyei is northern Sudanese land," Bashir said, adding he had given the green light to the northern army to respond to any "provocation" by the south, which also claims Abyei and plans to secede in July.

Analysts are watching how the south will react, fearing further north-south fighting could reignite a full-blown conflict that would disrupt the already fragile region.

Sudanese forces and and South Sudan separatist fighters fought for decades before a 2005 peace deal that also allowed southerners to vote overwhelmingly for independence in a referendum in January.

'War crimes'

The United Nations said on Tuesday it had asked South Sudan to investigate separate attacks on UN peacekeepers in Abyei last week by what appeared to have been southern security forces.

It said that such attacks amounted to war crimes.

"The available information and eyewitness accounts describing the assailants, including their uniforms, strongly suggests that the attackers were members of the southern Sudan police or military forces," Martin Nesirky, UN spokesman, told reporters in New York.

A monitoring group has presented satellite images which indicate evidence of "war crimes" committed by Sudan's northern army in Abyei region.

John Bradshaw, director of the Enough Project campaign group, part of the coalition backing the satellite work, said that "These images provide supporting documentary evidence of war crimes and crimes against humanity in Abyei."

The Satellite Sentinel Project, which provided the images, was set up by Hollywood star and rights activist George Clooney last year.

Khartoum accused the south of ambushing northern troops travelling in a convoy with UN peacekeepers on May 20 in the Abyei region. The south denied carrying out the attack.

UN officials said between 15,000 and 20,000 people had fled Abyei, many heading to Agok, just over the southern border.

"We are concerned ... about the grave humanitarian consequences of what's transpired in Abyei. There have been horrific reports of looting and burning," said Susan Rice, US ambassador to the UN, visiting the southern capital Juba.