Clashes between feuding tribes in the region of Abyei in Sudan have left at least 30 people dead.
The fighting came as the rest of South Sudan took part in a referendum, on independence from Khartoum.
There are conflicting details about the fighting which reportedly took place over three days.
The first day of voting in the rest of South Sudan was hailed internationally as a positive start.
The vote was part of a peace deal in 2005 which ended decades of civil war. The north has promised the south it will not block its plans to secede if that is the result of the vote, as widely expected.
At least 60% of the 3.8 million registered voters must take part for the referendum to be valid.
The BBC reports Abyei was due to hold a separate referendum on whether to join north or south Sudan, but this has been postponed indefinitely because of disagreements over eligibility.
Southern Sudan is one of the least developed areas in the world. But Abyei has oil.