The Intergovernmental Authority on Development (IGAD), a regional bloc for East Africa, extended the invitation on Wednesday to the two sides, led by President Salva Kiir and opposition leader Riek Machar.

The letter urges them to attend the May 2-3 talks in Addis Ababa to discuss "the implementation of the peace agreement and the way forward".

The peace deal, signed in September, put an end to much of the civil war that had ravaged the world's youngest nation since December 2013, killing 400,000 people and displacing four million.

But the formation of a transitional government combining both camps has not been forthcoming.

Machar -- who fled the capital Juba in 2016 when a previous peace deal fell apart -- has asked for a delay of six months, saying it was too unsafe for him to return.

Kiir last weekend called on Machar -- his former vice president and an erstwhile ally in South Sudan's struggle for independence, to return to Juba from neighbouring Sudan.

Machar's continued absence would destroy the current peace accord, he warned.

The transitional government is meant to be formed by a May 12 deadline, with Machar once again taking up the post of vice president.

Government and opposition representatives confirmed to AFP they had received IGAD's invitation and they had the intention of participating in the talks.

But there were differences in what they expected.

For the government, the discussions needed to focus on "arrangements that will lead to the preparations of the formation of the transitional government in May," said a spokesman for South Sudan's foreign ministry, Mawien Makol.

But the head of one opposition group, Peter Mayen Majongdit, said the Addis talks should decide "whether the government will be formed or not" in May.

Political turmoil in neighbouring Sudan, where longtime president Omar al-Bashir -- a prime architect of the South Sudan accord -- was ousted two weeks ago, has raised fears that the peace deal could be weakened.