JUBA, July 5 (Xinhua) -- A coalition of South Sudanese opposition political parties on Thursday threatened to pull out of the ongoing peace talks in Khartoum, Sudan over what it called imbalance in a proposed power sharing formula.

Peter Mayen, head of the eight-member South Sudan Umbrella Coalition of Political Parties for Peace and Democratic Transformation, said they reject the revised power sharing pact which gives the opposition parties one ministerial position and 17 parliamentary seats.

"Some parties are actually thinking about withdrawing from the process itself because generally we should expect a better proposal from time to time. What we have been seeing is a shift from position one to the other side," Mayen told UN-backed radio Miraya from Khartoum.

Last week, President Salva Kiir and rebel leader Riek Machar signed a cessation of hostilities agreement in Khartoum and pledged to end more than four years of fighting.

But the pact was broken hours after it took effect and the parties have been trading blame games on one another.

South Sudan descended into civil war in late 2013, and the conflict has created one of the fastest growing refugee crises in the world. The UN estimates that about 4 million South Sudanese have been displaced internally and externally.

A peace deal signed in August 2015 between the rival leaders under United Nations pressure led to the establishment of a transitional unity government, but was shattered by renewed fighting in July 2016.

Sudan is continuing with mediation efforts initiated by East African regional bloc, Intergovernmental Authority on Development (IGAD) in a bid to find political solution to the South Sudan conflict.

The Sudanese mediation team on Wednesday availed a revised version of the power sharing formula which the warring parties had rejected in May.

"Whenever people expect better things, things get worse than the matter, but generally none of the parties agreed to the proposal," Mayen added.