sudan rocket
A rocket fired by government forces cruises over a hut burnt by a previous rocket along the frontline at Mathiang, South Sudan Reuters

Western diplomats have threatened sanctions against wartorn South Sudan if violence continues.

US and EU envoys said Washington and Brussels were both considering "targeted measures" against individuals. No further details were given.

Warring sides in the world's youngest country reached a temporary ceasefire in January but peace negotiations were abruptly interrupted by rebels who sparked new violence.

The previous peace talks had plunged into stalemate as the government refused to accommodate opposition requests.

The rebels had asked for the release of 11 political prisoners, who had been accused of an attempted, and the withdrawal of Ugandan troops, who were said to be supporting government forces.

The South Sudanese government agreed to release only seven political prisoners The remaining four would undergo trial, it said.

"Patience is beginning to run a little thin with the parties to this conflict who think that they can toy with their own agreements and with their neighbours and eventually at the cost day by day of lives of their own citizens," Alexander Rondos, representing the US envoy to the Horn of Africa, said.

New negotiations mediated by the east African Intergovernmental Authority on Development (IGAD) were due to resume in Addis Ababa after a two-week break but diplomats were unclear if both sides would attend.

"If the government and any other actor tries to undermine the peace process and rebuff the IGAD heads of state, they will face consequences," said the EU envoy to Sudan and South Sudan Donald Booth on behalf of the EU, US and Norway.

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The conflict in South Sudan started in December after President Salva Kiir, an ethnic Dinka, accused his former vice-president Riek Machar, an ethnic Nuer, of an attempted coup.

Machar denied the allegations and in turn accused the president of planning a purge.

An ethnic conflict erupted in the wake of the accusations. More than 2,000 people have been killed and hundreds of thousands displaced.

South Sudan civilians have been subjected to rape, extrajudicial arrests, torture and pillage by warring factions, according to a UN report.

The fighting has hit hard the country's oil industry.