The talks in the Ethiopian capital are aimed at ending three weeks of fighting that have already left thousands dead in the world's newest nation, and US Secretary of State John Kerry urged the two sides not to use the talks as a "gimmick" for buying time.
"Negotiations have to be serious, they cannot be a delay, (a) gimmick in order to continue the fighting and try to find advantage on the ground at the expense of the people of South Sudan," Kerry told reporters while on a visit to Jerusalem.
Regional peace brokers appeared to be struggling to prevent a breakdown, amid heavy fighting overnight inside Juba and more clashes elsewhere in the country, plus key differences over the fate of rebel officials detained by the government of President Salva Kiir.
The spokesman for South Sudan's government delegation, Information Minister Michael Makuei, struck a confrontational tone by again accusing rebel leader Riek Machar of having started the fighting by attempting a coup.
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"We are being told to negotiate with the rebels. But any rebels who have fallen in our hands will have to answer why he or she decided to take up arms against a democratically-elected government," he said, asserting that "nobody is above the law."
Ethiopian government spokesman Getachew Reda said IGAD -- the East African regional bloc brokering the talks -- was trying to convince South Sudan's government to release 11 detainees, many of them former senior government officials, as a "goodwill gesture."
The United States, which was instrumental in helping South Sudan win independence, has also urged South Sudan's government to "release political detainees immediately" so that they can take part in the negotiations.