They were allies in the struggle for statehood, but their subsequent rivalry turned into war from December 2013
South Sudan President Salva Kiir on Saturday urged opposition leader Riek Machar to return home "urgently" to help form a government of national unity.
A failure by the former vice president to do so would destroy any hope for peace in the war-torn country, he said.
"Even now, it's not too late," Kiir told reporters. "I am inviting Dr. Riek Machar to urgently return to Juba so that we can work together to expedite the process of forming the revitalized Transitional Government of National Unity".
A peace deal signed by the parties of the country's six-year-old conflict calls for the formation of a government of national unity on May 12.
Machar, who is a signatory of the agreement, on Thursday asked for a delay of six months, saying it was not safe for him to return to Juba. According to leaders of his party he is in Khartoum, the Sudanese capital.
Machar "will not come to Juba without security arrangements," said Puot Kang Chol, representing Machar's SPLM-IO rebel group at the National Pre-transitional Committee (NPTC), the body charged with implementing the peace agreement.
But Kiir said that "we should hold the interest of our people above and beyond any personal fears, resentments and interests.
"Any delay to the formation of the government would crush the hopes of our people for peace," he warned.
Machar fled Juba in 2016 under fire from Kiir's troops after a previous peace deal fell apart, prompting clashes between both men's forces.
The September peace deal has ended most of the fighting, but progress towards a unified army and the security of the capital have been slow.
The deal is the latest effort to end years of conflict between both men that has plunged their country into a civil war marked by atrocities, including murder and rape, motivated by ethnic differences.
Predominantly Christian, South Sudan is the world's youngest country, gaining its independence from Sudan in 2011 after 22 years of conflict.
Kiir and Machar were allies in the struggle for statehood, but their subsequent rivalry turned into war from December 2013.
The government has asked foreign donors to fund much of its $285-million budget to implement the peace deal. But diplomats say it has yet to take important budgetary steps and improve transparency.
Kiir spokesman Ateny Wek Ateny told Eye Radio this week that other opposition figures were already in Juba, and this should tell Machar "that his security will be taken care of while the government is formed".