Machar was meant to return to South Sudan in May and join a power-sharing government as vice president with President Salva Kiir after the signing of a peace deal in September last year.

The deal is the latest effort to end almost six years of conflict which erupted due to a fallout between the two men in 2013. Observers, though, have been warning that implementation of the deal has stalled.

"We are saying extend the period for more months, say six," said Stephen Kang Chol, representing Machar's SPLM-IO rebel group at the National Pre-transitional Committee (NPTC), the body charged with implementing the peace agreement.

Machar "will not come to Juba without security arrangements," he said.

Machar fled Juba in a hail of gunfire in 2016 after the collapse of a previous peace agreement saw his troops clash with Kiir's.

The latest deal has largely stopped fighting in the country.

However, crucial steps such as establishing a unified army and discussing security control of the capital have yet to take place.

"We have not done even training of few people, and it is a pre-requisite for forming the government," said Chol.

The independent radio station Eye Radio reported Wednesday that 3,000 opposition and government troops were being trained together, although Chol denied this.

The other key issue yet to be addressed is the contentious matter of internal boundaries.

"You cannot form a government without knowing the number of states," said Chol.

"Basically what we are saying is extend the time, provide the resources ... and we will have the government in place."

The government has asked foreign donors to fund much of its $285-million (252-million-euro) budget to implement the deal. But diplomats argue they have yet to take important budgetary steps and improve transparency.

Kiir's spokesman Ateny Wek Ateny told Eye Radio that other opposition figures were already in Juba, and this should tell Machar "that his security will be taken care of while the government is formed."