JUBA, South Sudan — Secretary of State John Kerry said Friday that he had secured a commitment from Salva Kiir, South Sudan’s president, to open talks with his rebel opponent as early as next week.

The talks, which would take place in Addis Ababa under the auspices of Ethiopia’s prime minister, represent the Obama administration’s latest bid to halt the fighting that has killed thousands of people, displaced more than a million and is ripping apart a nation whose independence was championed by the United States.

Mr. Kerry said that Riek Machar, the rebel leader, had previously signaled to Ethiopia’s prime minister that he was also prepared to attend. In a call Friday night, Mr. Kerry also urged Mr. Machar to participate in the meeting, The talks would focus on implementing the cease-fire that was agreed to by the two sides in January but was widely ignored, as well as on the formation of a possible transitional government.

Providing new details of the parallel effort to provide security for the thousands of civilians trapped in the fighting, Mr. Kerry also said he expected that some 2,500 African troops would be sent to buttress the beleaguered United Nations effort here.