"2014 is key to Afghanistan's stability. The world cannot afford an unsuccessful Afghan transition," said the United Nations High Commissioner for Refugees at the end of a two-day visit to Iran.
Some five millions Afghans driven by war, oppression and poverty have crossed the border into Iran and Pakistan in the past three decades, seeking better lives and jobs.
Almost one million Afghans are illegal immigrants in Iran while some 840,000 have been able to register as refugees, according to police figures.
Non-governmental organisations and the UNHCR say Afghanistan -- whose economy is mostly supported by foreign aid -- is ill-prepared to host a large return of refugees after the departure of NATO forces.