U.S. President Barack Obama has urged South Sudan to avoid military confrontation with neighboring Sudan.

The White House says Mr. Obama spoke by phone Monday with South Sudan's president, Salva Kiir. It says Mr. Obama expressed concern about the growing tensions between South Sudan and Sudan, especially violent clashes on their border.

It says the president urged Mr. Kiir to ensure South Sudan's military is not involved in nor supports fighting along the border with Sudan, especially in Sudan's Southern Kordofan state.

Sudan has accused South Sudan of supporting rebels in Southern Kordofan and the state of Blue Nile, charges South Sudan has denied. The south accuses the north of conducting airstrikes on its territory.

Sudan's and South Sudan's defense ministers met with African Union mediators Monday in the Ethiopian capital, Addis Ababa.

The talks are aimed at implementing a joint security arrangement to monitor and investigate cross-border hostilities that threaten to plunge the arch-rivals into war.

Each side is accusing the other of starting the fighting last week around the northern oil center of Heglig, which triggered a wave of clashes along the border.

President Obama Monday emphasized the importance of the two countries reaching an agreement on oil.

In January, South Sudan shut down oil production that is the lifeline of both country's economies. The south accuses the north of charging excessive fees to use northern pipelines.