CAIRO Aug 23 (Reuters) - South Sudan opposition leader, Riek Machar, is in Khartoum for medical treatment, Sudan's Information Minister Ahmed Bilal Osman told the state news agency SUNA on Tuesday.

President Salva Kiir sacked Machar from his post as vice president after a revewal last month of fighting between forces loyal to the long-time rivals that forced tens of thousands of people to flee.

Machar withdrew to the bush and was picked up this month by U.N. peacekeepers in Democratic Republic of Congo with a leg injury.

An opposition spokesman had said the injury was not serious enough to require medical attention. However, on Tuesday, Sudan said he was having treatment there.

"Sudan has received, lately, Dr. Riek Machar, for pure humanitarian reasons, especially his need for treatment and medical care," Osman said.

"Dr. Riek Machar's health is stable currently and he will remain in the country under comprehensive healthcare until he leaves to a destination of his choice to complete his treatment," he added.

Machar's spokesman in Nairobi, James Gatdet, could not immediately confirm he had travelled to Khartoum.

South Sudan secured its independence from Sudan in 2011, but by December 2013 the longtime political rivalry between Kiir, an ethnic Dinka, and Machar, a Nuer, had led to civil conflict that often followed ethnic lines.

The two sides reached a peace deal in August 2015. Under the deal, Machar returned to Juba in April to resume his role as vice president. But fighting flared last month and he was then sacked.

During a visit to Kenya, U.S. Secretary of State John Kerry urged South Sudan's leaders to "get the job done" by fully implementing the peace deal or face a U.N. arms embargo and sanctions.

The U.N. Security Council voted on Friday to authorise sending an extra 4,000 troops to the country to bolster the existing U.N. mission, which South Sudan said it was considering. (Reporting by Ali Abdelaty; Writing by Asma Alsharif; Editing by Alison Williams)