South Sudan's government and rebels are wrangling over weaponry and therefore still at odds over the terms for allowing rebel leader Riek Machar to return to the capital, a government negotiator said on Thursday.

On Wednesday, the two sides said a deal had been reached on Machar's delayed return to form a unity government.

However, government negotiator Michael Makuei now says the government cannot allow the rebels to bring some of the weapons proposed including rocket-propelled grenade launchers.

"The problem is not the (rebel) SPLM-IO any more, the delays are on the government side," said SPLM-IO negotiator Taban Deng Gai.

The chairman of the Joint Monitoring and Evaluation Commission (JMEC), Festus Mogae, said after a hastily convened meeting aimed at resolving the dispute that a proposal had been presented to the government and SPLM-IO.

"The SPLM-IO accepted the proposal but the government gave their reservation," he said.

JMEC, which includes Western powers, African representatives and others, had proposed allowing Machar to bring 195 members of his forces, part of a quota agreed in a peace deal, and a limited amount of arms.

The two South Sudan rivals have strained the patience of international mediators, frustrated by continued wrangling after the peace deal was signed in August to end more than two years of fighting in a nation that gained independence in 2011.

Machar, who will be First Vice President in a transitional government with President Salva Kiir, was due to fly back early this week.

But his return was postponed, the latest in a series of such delays, in a dispute over how many soldiers and weapons he could bring.

Mogae said there would be further talks on Friday and, if there was no breakthrough, the matter would be sent to the UN Security Council and African Union peace and security council for "an appropriate response".

The United States and UN Security Council have voiced concern over the latest delay to Machar's return after a conflict in which thousands have been killed and more than two million forced to flee their homes.

READ: US criticises Machar delay in return to Juba

Kiir's sacking of Machar as his deputy in 2013 precipitated the crisis that led to a conflict in December 2013.

Fighting has often run along ethnic lines, pitting Kiir's dominant Dinka ethnic group against Machar's Nuer.

The conflict has hammered the economy and left swathes of the 11 million population without enough food.

Oil production, South Sudan's main source of revenue, has tumbled as oil fields have been cut off and global prices have dropped.