"It was exhaustion. He simply was not feeling well but there was nothing specific," the minister of cabinet affairs in the oil-producing region, Kosti Manibe, told Reuters.

"He did have some check-up there (in Nairobi). There was nothing wrong with him. He just needs to rest ... People should not be worried at all."

Manibe said Kiir left the southern capital Juba last week and was due to return on Sunday.

The south was on Friday exactly six months away from the scheduled start of a referendum on whether the underdeveloped territory should split away and form an independent country.

Kiir's vice president Riek Machar and other senior leaders were due to meet their northern counterparts in Khartoum on Saturday for the formal start of talks on how to share out oil revenues after the plebiscite and other issues.

Manibe said the president's check-up would not delay any of the discussions.

Tabitha Butrus Shokai, a member of the political bureau of Kiir's Sudan People's Liberation Movement (SPLM), said the president had gone to Nairobi for a general check-up and was in good health.

The south secured the referendum in a 2005 peace deal that ended more than two decades of civil war with the north and created a semi-autonomous southern government.

More than 700 Juba residents held the second in a series of marches on Friday, calling on people to take part in the referendum and vote for separation.

Marchers held banners and wore t-shirts marked with slogans including "Yes for Separation" and "Let My People Go: Exodus 8.1" as they marched from a football stadium into the center of the capital.

"The referendum is going to be held. There is no way out," Daniel Awet, the deputy speaker of the southern parliament, said at the march. "We have to prepare ourselves. We have been fighting for this day for 24 years."