In a country awash with guns, a faltering peace deal aimed at ending over two years of intense civil war in South Sudan came down to a dispute over just two dozen weapons.

The issue, while apparently minor, reflects the huge mistrust between the rival leaders, and is a sign of the massive challenges faced when -- or if -- rebel chief Riek Machar finally returns to the city and forges a unity government.

"Both sides are very deeply suspicious," said veteran South Sudanese journalist Alfred Taban, editor of the independent Juba Monitor newspaper.

"But people are just sick of war. People are looking for some sanity."

Machar was due to return to the capital on Monday to take up the post of first vice president alongside arch-rival President Salva Kiir, and his failure to arrive has put an August 2015 peace agreement in jeopardy.

The latest stumbling block to his return concerned the number of machine guns and rocket-propelled grenades that rebel troops protecting Machar would be allowed to carry.

Diplomats said it was "almost unbelievable" that a deal to end a war in which tens of thousands have died could rest on such a difference.

Rebels finally accepted proposals by international mediators that they could hold 20 of each weapon, but when talks broke late Thursday the government insisted on only seven.

The government accepted the deal on Friday, after threats of being reported to the UN Security Council.

UN chief Ban Ki-moon urged Machar to return to Juba "without delay", while the US, UK and Norway in a joint statement demanded he return by Saturday.

However, Minister of Information Michael Makuei said Monday was more likely, as ceasefire teams must verify the weapons first.

But diplomats also noted gloomily that while Machar's return is the "best chance yet", it only sees the country go back to the status quo that existed before his July 2013 sacking that precipitated the war.

"Now the economy is also broken and there's two years' worth of division on top," one diplomat said.

J. Peter Pham from the Washington-based Atlantic Council think tank, called the deal a "forced marriage that the international community is dragging both men kicking and screaming towards" amid fear of possible sanctions.

South Sudan's civil war began in December 2013 when Kiir accused Machar of plotting a coup.

The conflict has torn open ethnic divisions and been characterised by human rights violations.

- Warlike peace -

It has included the abduction and rape of thousands of women and girls, massacres of civilians, recruitment of child soldiers, murder, mutilation and even cannibalism.

The government on Monday appeared ready to receive Machar, but after he failed to appear, the red carpet laid out at Juba airport was rolled up and a box of white doves representing peace taken away.

"Soon he will come, we pray," said the woman carrying the birds. She wore a specially printed cap with a smiling picture of Machar alongside the slogan "With peace we all win".

Machar is a former rebel leader turned deputy president who was fired, became a rebel leader again and has now fought his way back to the vice presidency.

"It's only hypocritical to think that his return and creation of a government he is part of will be the panacea for the country's current troubles," said Jok Madut Jok, a former top government official who heads the Sudd Institute think tank.

The threat of violence at a local level remains enormous, with multiple militia forces unleashed and out of control.

Machar and Kiir are decades-old rivals and even if they can work together both must also rein in powerful hardline field commanders.

- Guns and mistrust -

Both sides have shown bad faith throughout tortuous negotiations hosted by regional governments, and only signed the peace agreement under huge pressure and the threat of sanctions, including from the US.

Mutual suspicion lies behind the squabbling over weapons that has delayed Machar's homecoming.

A 1,370-strong armed rebel force has already arrived in Juba as part of the peace deal while government forces say they have pulled out all but 3,420 of their troops, according to the agreement.

All other soldiers have to remain at least 25 kilometres (15 miles) outside the capital.

At one of three rebel camps in Juba, rebels pointed towards the government camp five minutes walk away and well within sight and range.

"Our people tell us, do not go back to war again," said rebel Colonel Par Dang, as his 500 troops built thatch shelters in the scrubland.

"We are all South Sudanese, and even though we lost thousands, for those who survived we must work together again."