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. In a striking gesture meant to urge the rivals to finally make peace, Pope Francis knelt and kissed their feet during a meeting at the Vatican earlier this month.
The opposition has expressed "serious concerns" about the agreement. It would be a "recipe for disaster" if Machar returns without security measures in place, his wife, Angelina Teny, has said.
The committee charged with overseeing the peace deal's initial stages will consider the six-month extension request on Wednesday, according to the opposition. The committee is made up of members of the government and various opposition parties.
This latest peace deal has been marked by delays and continued fighting in parts of the country, with key aspects yet to be implemented. South Sudan's internal boundaries have not yet been drawn. A unified national army has not been formed.
Alan Boswell, senior analyst with the International Crisis Group, warned that the deal would "look very flimsy if Kiir unilaterally forms a new government without Machar."
South Sudanese are already wary of possible violence next month, said a recent report by the Community Empowerment for Progress Organization, a local advocacy group. Without clear messaging from the parties' leaders the risk of citizens "panicking is high," it said.
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