The U.N. secretary-general is urging African Union states to address South Sudan's failure to meet a deadline to establish a transitional government of national unity.

Ban Ki-moon said through his spokesman Monday that forming a transitional government is an "essential step" in implementing a peace deal between South Sudan's government and rebels.

Ban called on South Sudan's parties to overcome their differences after failing to meet Friday's deadline. He asked African leaders to make the peace deal a priority at the AU summit this week that he plans to attend.

The two sides in South Sudan are deadlocked after President Salva Kiir issued a Christmas Eve decree dissolving the nation's 10 states and creating 28 new ones. The peace deal rests on power-sharing in the original states.