By Frances Harrison
BBC News, Tehran
More than 36,000 Afghans have been deported from Iran in the past 10 days, the UN refugee agency says.
Millions of Afghan refugees live abroad
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.
He denied that families had been divided in the deportation process and said Iran had allowed 4,000 of the Afghans they had rounded up to remain because of problems they faced.
The UN says 20,000 of those deported come from the south-eastern border town of Zabol.
They say that on the Afghan side of the border initially there was not enough food or water or health facilities to cope with the influx, but that has now been provided.
Iran is estimated to have up to one and a half million illegal Afghan workers who do the most dangerous and dirty jobs with little legal protection.
There are also just under a million registered refugees.
Tehran says some 300,000 of those refugees will be eligible to apply for work visas to return to Iran, if they first go back to Afghanistan.