KHARTOUM -- Four UN helicopters were shot at, probably by militias allied to north Sudan, in the central region in Abyei but the crews landed safely, a UN spokeswoman said on Wednesday.

A total of 14 rounds were fired when the helicopters took off on Tuesday, spokeswoman Hua Jiang said.

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North Sudan seized Abyei at the weekend, forcing thousands of people to flee and raising tensions ahead of southern secession planned for July.

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, the border area's main settlement, on Saturday, sparking an international outcry as well as fears of new north-south fighting.