Troops from northern and southern Sudan have reportedly clashed in the disputed Abyei region, as talks to ease tensions between the sides continue in Ethiopia.

A southern army spokesman, Philip Aguer, says the sides exchanged fire Wednesday on the Bahr al-Arab River, also known as the Kiir River, when northern troops tried to cross a bridge.

Aguer said there were casualties but did not say whether anyone was killed.

There was no immediate comment from north Sudan officials.

The African Union is hosting talks in Addis Ababa designed to end violence along Sudan's north-south border as the south gets set to declare independence next month.

The AU said Monday the sides had agreed in principle to demilitarize Abyei, which the north occupied last month. But VOA's Peter Heinlein, who is present at the talks, reports that deal appears to be unraveling.

Meanwhile, officials say a joint U.N.-African Union mission will head to Sudan's Southern Kordofan state Thursday in an effort to bring a halt to heavy fighting there.

The U.N. humanitarian affairs office says fighting between the northern army and local pro-southern fighters in Southern Kordofan has displaced 60,000 people.

VOA's Heinlein reports the mission to Southern Kordofan will be led by former South African president Thabo Mbeki, who heads a high-level AU panel on Sudan, and the U.N. secretary-general's special representative to Sudan, Haile Menkerios.

U.S. President Barack Obama has called on the north Sudanese government to stop what he called its “campaign of intimidation” along the north-south border.

In an audio message aired by VOA late Tuesday, Mr. Obama said the leaders of both north and south Sudan must live up to their responsibilities and agree to end the violence.

If they flout their obligations, Mr. Obama said they will face international pressure and isolation and will be held accountable for their actions.

Both north and south Sudan claim ownership of Abyei. The dispute there and in Southern Kordofan has raised fears of a renewed war in Sudan as the south prepares to declare independence July 9.

North and south Sudan fought a 21-year that ended with a 2005 peace deal. The south voted overwhelmingly to split from the north in a referendum in January.