South Sudan's President Salva Kiir has been rushed to hospital in Ethiopia's capital, Addis Ababa, after becoming ill, an official has told the BBC.
His hospitalisation has delayed a summit of regional leaders called to discuss the conflict in South Sudan, the official said.
Mr Kiir, 63, has led the country since its independence from Sudan in 2011.
He held talks in Addis Ababa on Wednesday with rebel leader Riek Machar to end the civil war which has killed thousands of people and displaced more than 1.5 million since December 2013.
The BBC's Emmanuel Igunza in Addis Ababa says Mr Kiir's office has not commented on his illness, but an official from the regional body Inter-Governmental Authority on Development (Igad) confirmed that he had been taken to hospital on Wednesday night.
Ethiopia's Prime Minister Hailemariam Desalegn was due to have chaired Thursday's summit.
Other regional leaders, including Uganda's President Yoweri Museveni and Kenyan President Uhuru Kenyatta, are also in Addis Ababa for the meeting.
President Kiir and Mr Machar are under pressure from Igad to form a unity government, but a major sticking point is the powers that a new prime minister will have, our correspondent says.
Mr Kiir sacked Mr Machar as South Sudan's deputy president in 2013 and later accused him of plotting a coup.
Mr Machar denied the allegation, but then marshalled a rebel force to fight government troops.