African Union leaders are trying to choose the head of the bloc's main decision-making body during a vote on the final day of a two-day summit.

The contest in Addis Ababa, Ethiopia pits incumbent AU Commission Chairman Jean Ping of Gabon against South African Home Affairs Minister Nkosazana Dlamini-Zuma. There was a deadlock after three rounds of voting, with neither candidate receiving the two-thirds required to win.

Leaders began the summit Sunday by electing Benin's President Thomas Yayi Boni as the AU's rotating head for the next year.

A dispute between Sudan and newly independent South Sudan, along with the crisis in war-torn Somalia, are key issues at the summit.

U.N. Secretary-General Ban Ki-moon warned Sunday the Sudan-South Sudan dispute has reached a “critical point” and has become a threat to the region's peace and security.

He also expressed concern about a growing humanitarian crisis along the disputed border that separates the two countries.

South Sudan has followed through on its threat to shut down its oil production. It controls more than 70 percent of the two countries' oil output but needs pipelines running through Sudan to get the oil to port.

The leaders of both countries held talks in Addis Ababa late last week to work on a deal to share oil revenues. But the negotiations fell through when South Sudan's president, Salva Kir, said he could not accept the terms for transit fees

The South claims the Khartoum government has illegally seized hundreds of millions of dollars worth of oil.