Shots have been fired at UN helicopters in the disputed Sudanese region of Abyei, the UN says.
A UN spokesman told the BBC that 14 shots had been fired at four helicopters but none had been hit.
Tension is high in Abyei, which was seized by northern troops at the weekend - a move condemned by the UN Security Council.
The region is also claimed by South Sudan, which is due to become independent in July.
Analysts fear the the dispute could reignite the north-south conflict, in which some 1.5 million people were killed.
The status of Abyei was left undecided in the 2005 peace deal and a referendum, due in January, on whether the area should be part of the north or south has been postponed indefinitely.
Aid workers say some 40,000 people have fled the fighting around Abyei - mostly southerners, heading further south.
A southern official said many were in a "miserable situation".
The UN believes militiamen from the Misseriya ethnic group were responsible for shooting at the helicopters.
The Misseriya are northern nomads and one of two groups to claim Abyei, along with the southern Dinka Ngok people.
The two groups often clash as their herds of cattle look for water and pasture.
The Misseriya were armed by Khartoum and used to attack the south during the civil war.
The BBC's James Copnall in Khartoum says the Misseriya are also being blamed for what the UN calls "burning and looting" in Abyei.
Reports suggest many Misseriya have arrived in the town since the northern armed forces took control of it on Saturday.
A Misseriya leader, Sadig Babo Nimr, told the BBC the accusations were "fallacious nonsense" and "against logic".
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