Addis Ababa (AFP) - A South Sudanese minister and opposition figure resigned on Monday saying a peace deal designed to heal the troubled young nation was dead, while calling for President Salva Kiir's unity government to leave power.

Lam Akol was agriculture minister in Kiir's administration and also announced he was quitting as longtime leader of the opposition Sudan People's Liberation Movement-Democratic Change (SPLM-DC) party.

"There is no more peace agreement to implement in Juba," Akol said at a press conference in the Ethiopian capital, Addis Ababa.

"All of us agree that the regime in Juba must change," he added.

Akol was the president's only serious challenger in a regional election held in 2010, the year before the semi-autonomous region known as Southern Sudan seceeded and gained full independence.

He has also long opposed rebel leader Riek Machar, whose forces clashed in recent weeks with government troops loyal to Kiir.

In December 2013, a skirmish in Juba between troops loyal to to Kiir and Machar degenerated into a ruinous civil war.

The latest in a series of deals designed to end the conflict was signed in August 2015.

It was under that agreement that Machar attained the position of first vice president and that 30 ministerial posts were distributed between the two and to other parties.

Machar has not returned to Juba since fighting broke in early July, and a week ago Kiir named another member of his SPLM/A (IO) party to replace him as first vice president.

On Monday Akol left open the possibility that he could join forces with Machar in some configuration after leaving his own party.

"We are consulting as I speak how to organise so that the opposition to the government is consolidated," he said.

"Since the agreement is dead and there is no free political space in Juba, the only sensible way to oppose this regime so as to restore genuine peace... is to organise outside Juba," Akol added.