South Sudan's president and rebel chief have met in Addis Ababa in a bid to end six months of civil war, agreeing to forge a transitional government within a 60-day deadline, Ethiopia's prime minster says.

President Salva Kiir and rebel leader Riek Machar met on Tuesday on the sidelines of a regional leaders' summit organized by the East African regional bloc the Inter-Governmental Authority on Development (IGAD), which is brokering the slow-moving negotiations.

"They agreed to complete the dialogue process within the coming 60 days on what how, when and who... (for) the formation of the transitional government," Ethiopia's Hailemariam Desalegn said, after the rare meeting between Kiir and Machar.

Mediators, frustrated by the failure of previous deals including two ceasefires, threatened tough action if the rivals once again ignore agreements made.

"Any attempt to stand in the way of peace will have consequences," Hailemariam warned.

The crisis in the young nation has already killed thousands and forced more than 1.3 million people from their homes.

Both Kiir and Machar had also agreed to "fully to commit themselves to already signed agreements," Hailemariam said.

"If they don't abide to this agreement... IGAD as an organization will act to implement peace," Hailemariam said, warning of possible "sanctions and punitive actions" without giving further details.

US envoy to South Sudan Donald Booth said the talks offered "the last, best chance for the warring parties to prove their commitment to holding their nation and their people together".

Bickering

It was the first encounter of the enemies since signing a May 9 ceasefire - broken within hours - and only their second meeting since the civil war began in mid-December.

"The time for military action to change the status quo on the ground has passed. It's time now to move forward," Booth said, at the summit opening.

Delegates for Kiir and Machar have been meeting in luxury hotels in the Ethiopian capital since January, with both sides bickering over the agenda and even the venue of discussions.

Previous rounds of peace talks have made little progress and been repeatedly delayed, and have so far cost more than $17m, IGAD officials said.

Earlier, mediators issued a stinging rebuke of both rivals, accusing them of seeking military victory rather than a negotiated end to the civil war.

Mahboub Maalim, IGAD executive secretary, said both Kiir and Machar were "stupid" if they thought they could win on the battlefield.

"If we want to apportion blame, it's theirs. I think sometimes they thought they could win on the ground militarily, something which is very stupid," Maalim said.

PHOTO CAPTION

South Sudan President Salva Kiir pictured at the opening of the first session of the National Legislature in Juba on June 2, 2014.

Aljazeera