[This report is also available as a radio story in Dari and Pashto on IRIN's Afghanistan Radio Page.]
Since 21 April about 85,000 Afghans have been deported to Afghanistan, Iran's deputy interior minister, Mohammad-Baqer Zolqadr, told the Iranian IRNA news agency on Saturday.
A joint UN and Afghan government assessment team visited a deportation centre on Thursday in the southwestern province of Nimruz, bordering Iran, where hundreds of deportees were in urgent humanitarian need.
"We agree that Iran is within its rights to deport illegal migrants, but it should ensure the process is carried out in an orderly and humane manner," said Fernando Arocena, Afghanistan country director of the International Migration Organisation (IOM) who headed the joint assessment team.
An Iranian diplomat in Kabul who spoke to IRIN on condition of anonymity rejected the criticisms: "Iran has not deported hundreds of thousands of illegal Afghan migrants all at once. The deportation process has been gradual".
However, the UN says "never before have families been deported in such numbers". Over 55,000 Afghans have been deported from Iran since late April, according to the UN Refugee Agency (UNHCR).
Some 920,000 Afghans are registered as refugees in Iran, which, according to the UNHCR, means they can live and work in the host country for the foreseeable future.
Many deportees accused Iranian security forces of inhumane treatment, including beatings and torture in detention.
A young man hospitalised in the western province of Herat said Iranian police beat him with batons until his arm broke.
"During my 48-hour detention I was given no food and was repeatedly insulted and harassed," said another young Afghan deportee.
Iranian diplomats in Kabul did not deny violence might have been perpetrated in a few cases. "We have asked Afghan officials to send us evidence which should prove allegations of violence against deportees. We will take appropriate measures," said a diplomatic source.
So far the government of Afghanistan has not submitted a formal petition to their Iranian counterparts about maltreatment of Afghan deportees, according to the Embassy of Iran in Kabul.
However, 130 cases of physical violence by Iranian security forces against Afghan citizens have been documented by deportees in Nimruz province, provincial officials said.
Many deportees say they could not collect their belongings and finalise their financial affairs in Iran due to the abrupt nature of the deportation process. Consequently they could not afford transportation to their final destination in Afghanistan.
In Zaranj city, the provincial capital of Nimruz, an emergency transition centre has been set up where deportees - said to be arriving at the rate of 400-800 per day - can stay for up to 48 hours.
The Afghan Red Crescent Society has provided about 20 tents and the provincial health department has established a temporary health clinic.
Although the local community has so far provided free food for the deportees, a confidential UN report has warned: "the goodwill of the local community is waning".
"Food assistance is needed immediately," said the report.
Ghulam Dastgeer Azad, the governor of Nimruz, told IRIN the scale of humanitarian demands among deportees is beyond his administration's capacity.
"If deportation continues and every day hundreds of people swarm into Nimruz, obviously neither local hospitality nor the provincial administration will be able to respond to the deportees' needs," Azad said.
Meanwhile the deportation of Afghans from Iran has become a sensitive political issue in Afghanistan. Last week MPs sacked two ministers for failing to tackle the issue adequately.