The government claims that the migrants take away 1.5m job opportunities from Iranians, but it is easy for them to find work because employers can pay them less money.
Al Jazeera's Laurence Lee said that people in Afghanistan believe that there is another motive for the expulsions.
|"When they arrested me from my work they handcuffed me and put plasters on my eyes and took me to jail ... I don't have any money at all and I live a long way away." |
"It's assumed in Afghanistan that the real reason is a protest by Iran against the Afghan government's support for American air bases inside Afghanistan and so Afghan migrant workers, according to that theory, are simply the softest of soft targets," he said.
Returning refugees told Al Jazeera that they had been seized by the Iranian authorities without notice, separated from their families and deported with little money.
"When they arrested me from my work they handcuffed me and put plasters on my eyes and took me to jail ... I don't have any money at all and I live a long way away," one man told Al Jazeera as he returned to Afghanistan.
Another said: "They left my wife alone with my six-year-old child. I don't know what happened to them, if he is alive or dead, and if he is dead who will answer for it."
Some legal refugees are given some protection in UN transit camps, but Lee said there were still stories of identity cards being ripped up and abuse by the Iranian authorities.
Josep Zapater of the UN high commission for refugees told Al Jazeera: "All the reports coincide that the authorities come for them in the evening, they don't have time to pack and the families are taken out and separated.
"Also, people have left important assets behind. Of the 263 families we interviewed in the first week they have left approximately half a million dollars just in cash."
Many of the expelled are treated little better on their return to their homeland.
At the border crossing visited by Al Jazeera returning refugees received just toothpaste, biscuits and water provide by a charity. They then had to find a way to travel back across Afghan provinces where aid organisations refuse to operate because of security concerns.