Four UN peacekeepers in Sudan have been wounded after their patrol was attacked in the disputed Abyei region along the north-south border.
The UN mission said the attack on its patrol in Goli village, 25km (15 miles) north of Abyei town, was unprovoked.
The BBC’s James Copnall says the incident comes at a surprising time.
On Sunday the north and south agreed to remove any unauthorised troops from Abyei, claimed by both sides, which was seen as a positive development.
The UN mission in Sudan, which brokered the withdrawal deal, said the peacekeepers attacked in Goli were from Zambia, and that one of them was in a critical condition.
Abyei is disputed by the Dinka Ngok, a southern ethnic group who are the permanent residents of the region, and the Misseriya, northern nomads who spend part of every year there seeking pastures for their cattle.
Since January there have been a series of bloody clashes between the groups.
They accuse each other using their security forces in the fighting, and of a build-up of troops near Abyei.
Our correspondent in the northern capital, Khartoum, says the fear is that if a solution is not found, Abyei could ignite a new north-south civil war.
South Sudan is preparing to secede from Africa’s biggest country in July, after 99% of voters backed independence in January’s referendum.
A draft version of South Sudan’s interim constitution explicitly claims Abyei is in the south.
Last month, President Omar al-Bashir threatened not to recognise the new state if it tried to claim Abyei.
The Abyei region was meant to have its own referendum on whether to join the north or the south in January, but agreement could not be reached on whether the Misseriya could vote.