More than 36,000 Afghans have been deported from Iran in the past 10 days
The move has virtually cleared the south-eastern Iranian border town of Zabol of its Afghan population.
Iran says it has collected 50,000 Afghans for deportation, and argues it is the right of every country to send home illegal workers.
Iran is estimated to have more than a million illegal Afghan workers, and slightly fewer registered refugees.
Although the numbers seem to have slowed down a bit in the past few days, the sheer scale and speed of the deportations has caused problems.
Iranian Interior Minister Mostafa Pour-Mohammadi told the state-run news agency that 50,000 illegal Afghans have been collected for deportation in the past 10 days.
But he does not say how many have actually been sent across the border.
But the UN says they know of about 36,000 deportations in the same period, and their figures come from monitoring the border crossing points.
Mr Pour-Mohammadi said Iran had warned the illegal workers in February that they would be sent back, but unfortunately they failed to prepare for leaving.
Iran defends plan to expel one million Afghans