South Sudanese rebel leader Riek Machar. File picture:Mohamed Nureldin Abdallah/ REUTERS.

Juba - South Sudan rebel leader Riek Machar is set to return to the country after more than two years in exile to take part in a peace ceremony, his spokesman said Tuesday.

"We are for peace and that is why our leader Riek Machar is travelling to Juba tomorrow on Wednesday ... for the celebration," Lam Paul Gabriel told dpa.

After five years of war, the rebels and the government signed a peace agreement in September after a lengthy negotiation process. However, questions remained over whether Machar, a former vice president of the country, would actually risk returning from self-imposed exile in South Africa.

The peace deal allows for the creation of a transitional government, with Machar reinstated as vice president, but Machar's party has expressed reservations about some points of the deal.

Also on Tuesday, a South Sudan army spokesman blamed Machar's rebels for an overnight ambush on a minibus in which five civilians were killed.

"We truly blame the rebels because their acts are anti-peace ... while the country is preparing for a celebration of peace," Lul Ruai Koang said.

South Sudan - the world's newest country - descended into civil war in 2013 after President Salva Kiir accused Machar, then his deputy, of plotting a coup.

Tens of thousands have been killed in the war, and about 4 million South Sudanese people have fled the fighting. Rights groups have reported gross human rights violations on both sides.

Previous agreements have collapsed after the warring parties failed to respect them, and numerous ceasefires have been broken.