The United Nations has confirmed that northern Sudanese warplanes carried out an air raid on a border area of south Sudan last week.
Spokesman Kouider Zerrouk says the U.N. mission in Sudan investigated the incident after the south's ruling party complained last Wednesday of aerial bombing along the border.
Zerrouk said the air attack took place in the vicinity of Timsaha in Western Bahr al-Ghazal state, and that no casualties have been reported.
The northern army did not immediately comment on the findings.
Southern Sudan is scheduled to hold a referendum on independence in less than a month. Most analysts expect the south to vote to break away from the north.
The Obama administration's special envoy to Sudan, Scott Gration, told reporters Monday that he believes the referendum will take place as planned on January 9.
However, Gration said he does not think a similar referendum can be organized on the same day in the disputed region oil-rich district of Abyei.
Disputes over who is eligible to vote have held up planning for that vote. Gration said it will take "a political solution" to decide Abyei's future.
The Comprehensive Peace Agreement in 2005 ended Sudan's 21-year north-south civil war and promised the referendum on southern independence.
Tension has been rising ahead of the vote, with each side accusing the other of violating the peace deal and building up troops along their shared border.