East African negotiators were struggling on Saturday to bring the warring South Sudanese parties into direct talks as the United Nations said it was boosting its presence in the country.

The chances of holding the first direct talks between representatives of President Salva Kiir and his rival, former vice president Riek Machar, appeared to be dimming as the sides were unable to agree on an agenda.

“We commit to peaceful negotiations where both sides’ opinions are respected,” Mr. Machar said on his Twitter feed. “Salva Kiir’s side aren’t willing to come to that term.” There had been hopes the two sides would meet directly this weekend, but the opposition to Mr. Kiir is demanding that prisoners be released as a condition for the talks. Mr. Machar’s loyalists also want a power-sharing deal to be discussed.

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