JUBA, June 28 (Xinhua) -- South Sudan has expressed optimism of reaching peaceful settlement of conflict after President Salva Kiir and rebel leader Riek Machar agreed to "permanent" ceasefire in the Sudanese capital.

Nhial Deng Nhial, President Kiir's political advisor, told journalists in Juba that the permanent ceasefire agreed upon by the two leaders will come into effect within 72 hours and the talks will continue on outstanding issues on governance and security arrangement mediated by the East African bloc IGAD.

"This time around we believe that these talks have been very successful they culminated into an agreement. The first point is permanent restoration of ceasefire that has been agreed and this comes into effect 72 hours from now," Nhial said in Juba on Wednesday evening after President Kiir returned to Juba from Khartoum.

President Kiir and Machar, the leader of the main rebel group the Sudan People's Liberation Movement/Army-In Opposition (SPLM/A-IO), were invited for face-to-face talks by the Sudanese President Omar al-Bashir in Khartoum in the aftermath of the two leaders meeting in Addis Ababa.

Nhial disclosed that the South Sudan government and some of the IGAD Heads of States agreed to work out modalities on ensuring the permanent ceasefire holds, contrary to previous agreements that have been violated on several occasions.

He also urged IGAD and the African Union (AU) to effectively oversee the ceasefire monitoring and verification process to avoid past violations of the ceasefire.

"It is noteworthy that it is an opportunity to have a mechanism that actually works because the problem that has failed us all this time on peace agreement several of them that have been negotiated and agreed have not worked," he said.

"The verification mechanism has not been strong enough we believe that this new verification mechanism by IGAD and AU should have a strong mechanism to make sure that the ceasefire holds," he said.

Nhial also revealed that South Sudan and Sudan discussed resumption of oil production that will involve repairing of damaged oil fields in the northern oil producing regions of Unity and Heglig oil field in Upper Nile.

"There has been discussion on how to repair the Heglig oil field. Each side will independently do assessment on rehabilitation of the oil fields," Nhial disclosed.

South Sudan descended into civil war in late 2013, and the conflict has created one of the fastest growing refugee crises in the world.

A 2015 peace agreement was shattered when the warring parties renewed fighting in July 2016 in the capital, forcing rebel leader Riek Machar to flee into exile.

The UN estimates that about 4 million South Sudanese have been displaced internally and externally.