He had been due on Monday to take up the post of first vice-president in a new unity government.

This is a key part of the deal aimed at ending more than two years of conflict.

The US said it was disappointed by Mr Machar`s "wilful decision" not to abide by his own commitments.

His team said the delay was caused by logistical and administrative issues and that he planned to return on Wednesday.

A statement from the government on Tuesday had said Mr Machar`s return was delayed as "he wanted to come with an arsenal of arms... anti-tanks, laser guided missiles and heavy machine guns".

Chairman of the regional monitors, Botswana`s former President Festus Mogae, urged both sides to "ensure that the spirit of reconciliation, compromise and dialogue embodied by the agreement" be protected.

This view had been echoed by US State Department spokesman John Kirby, who said that the US and other partners had gone to great lengths to facilitate Mr Machar`s return.

"His failure to go to Juba despite efforts from the international community places the people of South Sudan at risk of further conflict and suffering," he said.

It also undermined "the peace agreement`s reform pillars, which are demilitarising South Sudan, injecting transparency of public finances, and pursuing justice and reconciliation, all of which offer South Sudan a chance for renewal," the spokesman added.

The civil war broke out over tensions between President Salva Kiir and Mr Machar, who was sacked as vice-president in July 2013.

Then, in December 2013, Mr Machar was accused of trying to organise a coup. He denied the accusation but it set off a round of tit-for-tat killings, which developed into a full-blown conflict.

The government and the rebels have been slow to implement the peace deal, with reports of ceasefire violations on both sides.

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