Rebel troops in South Sudan have completed their return to the capital Juba as part of a peace deal, days ahead of the expected arrival of their commander, ceasefire monitors said Monday.

The 1,370-strong force of soldiers and police were flown to Juba to ensure security for rebel chief Riek Machar — named as vice-president in February — who is due to arrive in Juba next week.

The rebels were brought to Juba on United Nations and chartered airplanes "as required by phase one of the transitional security arrangements plan," the Joint Monitoring and Evaluation Commission (JMEC) said.

JMEC was set up by the regional IGAD bloc to ensure a repeatedly broken and delayed August 2015 peace deal is implemented.

Machar has said he will arrive in Juba on April 18 to form a unity government with President Salva Kiir, returning for the first time since he fled the capital in December 2013 when civil war broke out.

READ: Machar to return to Juba on April 18

However, rebel spokesman Mabior Garang on Sunday accused "hardliners" in the government of trying to undermine the peace process, including boosting government troop numbers in the capital.

Mr Garang said the rebels "find this situation unacceptable and reserve the natural right of self-defence", but said they remained committed to the peace process.

Tens of thousands have been killed in a war marked by atrocities, with over two million forced from their homes, and over six million in need of emergency food aid.

The conflict now involves multiple militia forces driven by local agendas or revenge, who pay little heed to paper peace deals.

JMEC chief Festus Mogae has already warned that "formation of a new government will not in itself be a panacea."