Nearly 450,000 Afghans left Iran since June 1: IOM
The influx comes as the country is already struggling to integrate streams of Afghans who have returned under pressure from traditional migrant and refugee hosts Pakistan and Iran since 2023.
The country is facing one of the world's worst humanitarian crises after decades of war.
This year alone, more than 1.4 million people have "returned or been forced to return to Afghanistan", the United Nations refugees agency UNHCR said.
In late May, Iran ordered undocumented Afghans to leave the country by July 6, potentially impacting four million people out of the around six million Afghans Tehran says live in the country.
Numbers of people crossing the border surged from mid-June, with some days seeing around 40,000 people crossing, UN agencies have said.
From June 1 to July 5, 449,218 Afghans returned from Iran, a spokesman for the International Organization for Migration told AFP on Monday, bringing the total this year to 906,326.
Many people crossing reported pressure from authorities or arrest and deportation, as well as losing already limited finances in the rush to leave quickly.
Massive foreign aid cuts have impacted the response to the crisis, with the UN, international non-governmental groups and Taliban officials calling for more funding to support the returnees.
The UN has warned the influx could destabilise the country already grappling with entrenched poverty, unemployment and climate change-related shocks and urged nations not to forcibly return Afghans.
"Forcing or pressuring Afghans to return risks further instability in the region, and onward movement towards Europe," the UN refugees agency UNHCR said in a statement on Friday.
Taliban officials have repeatedly called for Afghans to be given a "dignified" return.
Iranian media regularly reports mass arrests of "illegal" Afghans in various regions.
Iran's deputy interior minister Ali Akbar Pourjamshidian said on Thursday that while Afghans illegally in the country were "respected neighbours and brothers in faith", Iran's "capacities also have limits".
That the ministry's return process "will be implemented gradually", he said on state TV.
Many Afghans travelled to Iran to look for work, sending crucial funds back to their families in Afghanistan.
"If I can find a job here that covers our daily expenses, I'll stay here," returnee Ahmad Mohammadi told AFP on Saturday, as he waited for support in high winds and dust at the IOM-run reception centre at the Islam Qala border point in western Herat province.
"But if that's not possible, we'll be forced to go to Iran again, or Pakistan, or some other country."