South Sudan's warring sides have continued peace talks amid ongoing clashes in the African country.

The delegates, representing South Sudan’s President Salva Kiir and his former deputy Riek Machar, gathered for the talks in the Ethiopian capital Addis Ababa on Monday, while fighting resumed between militias and deserting soldiers in the country’s Upper Nile state.

An aid worker, sheltering in UN compounds, was shot dead by the gunmen.

The United Nations Mission in South Sudan (UNMISS) expressed concern over "the deteriorating security situation" in Upper Nile, where aid workers are trying to help over 125,000 refugees from neighboring Sudan.

"UNMISS strongly condemns the targeted killing of civilians by members of this militia," it said.

Mediators from the Intergovernmental Agency on Development (IGAD) -- the East African bloc brokering the talks -- have set an August 10 deadline for the creation of a transitional government and implementation of a truce.

"This session of negotiations must make progress, we must end the war," chief mediator Seyoum Mesfin said ahead of the talks.

The conflict in South Sudan "will have serious consequences ... those that insist on continued fighting must be held accountable," Mesfin added.

South Sudan plunged into violence in December 2013, when fighting erupted between troops loyal to Kiir and defectors led by Machar around the capital Juba.

The conflict soon turned into an all-out war between the army and defectors, with the violence taking on an ethnic dimension that pitted the president’s Dinka tribe against Machar’s Nuer ethnic group.

The clashes left thousands of South Sudanese dead and forced around 1.5 million people to flee their homes in the world’s youngest nation.

South Sudan gained independence in July 2011 after its people overwhelmingly voted in a referendum for a split from the North.