Tens of thousands of South Sudanese have been killed and millions displaced since the world's youngest country fell into a civil war less than three years after it seceded in 2011.
Urging crowds of refugees at Al-Nimir camp in the Sudanese state of East Darfur to "be strong and hopeful", Filippo Grandi said it was about time leaders of South Sudan ended a war that continued to rage on.
"I must confess that it may be a long term exile" for the refugees who continue to flee from their country every day, Grandi told AFP as he toured the camp, where about 5,000 South Sudanese have taken refuge.
Grandi said the refugees had to remain hopeful of returning to their country, but a lot depended on when South Sudan becomes stable.
"That hope depends on the action first and foremost of the leadership of South Sudan and the opposition," he said.
"They have to start behaving responsibly and thinking of their own people and not only of themselves."
South Sudan's civil war began in December 2013 when President Salva Kiir accused his former deputy, Riek Machar, of plotting a coup. Since then the war has spread across the country, sweeping up other ethnic groups and local grievances.
Overall the refugee population from South Sudan has reached to about two million, of which more than 430,000 have taken refuge in Sudan, the United Nations says.
Grandi praised Khartoum for opening several "human corridors" to deliver aid directly from Sudan to areas of South Sudan, and for hosting hundreds of thousands of refugees.
However, he said it was time to develop "new models" for aiding refugees, rather than just keeping them in camps.
"For how long can you support these camps?" he said as the United Nations and other aid organisations face increasing financial crunch.
It would be better to support the local economy and infrastructure, he said, as that would "benefit the refugees also".
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