South Sudan's opposition leader Riek Machar says he will return to the capital on April 18 to form a transitional government with President Salva Kiir after their acrimonious split ignited more than two years of war.

The oil-exporting country, which gained independence from Sudan in 2011, descended into chaos in December 2013 when a political row between Kiir and his then-deputy Machar spiralled into a vicious armed conflict that prompted a cycle of retaliatory killings along ethnic lines.

Under pressure from international powers, the two men signed an initial peace deal in August 2015 and agreed to share ministerial positions in January. Yet, the agreement has broken down repeatedly.

"I am therefore confirming the date of my arrival to be April 18 and thereafter form with President Kiir the Transitional Government of National Unity and hold the Transitional National Council of Ministers," Machar said on Thursday in a letter to the head of the body monitoring the implementation of the peace deal.

 
Forces allied with Machar gather outside South Sudan's capital Juba on Thursday [Jok Solomun/Reuters]  

On Wednesday, the UN mission in South Sudan said it had helped Machar's SPLM/A group transport more than 800 military and police officers to the capital Juba, including two of its generals.

Machar said in February that a condition to his return to Juba and taking up his old position of vice president was the demilitarisation of the capital, and that some of his soldiers be allowed to return with him.

Tens of thousands have been killed in the fighting that has displaced more than two million people from their homes.

The conflict has also hit hard the country's economy. South Sudan’s currency has weakened, inflation has skyrocketed, and oil revenues have dropped because of falling production and faltering world prices.