TEHRAN, Young Journalists Club (YJC) - Opposition deputy chairman Henry Odwar told The Associated Press on Saturday that the extension is needed because security arrangements are not yet adequate.
South Sudan's government rejects the idea of an extension, further raising concerns among observers that the peace agreement signed in September could fall apart.
The deal ended five years of civil war that killed nearly 400,000 people and sent millions fleeing.
There could be a "constitutional vacuum" if opposition leader Riek Machar does not return to South Sudan as scheduled to form the transitional government that is meant to culminate in elections, government spokesman Ateny Wek Ateny said.
May 12 is the deadline for Machar to return and once again serve as President Salva Kiir's deputy, an arrangement that more than once has ended in gunfire.