Iran has long been a draw for people seeking greater job opportunities as a humanitarian crisis grips Afghanistan, and as a transit route to destinations further afield.

But since US and Israeli strikes on Saturday triggered a regional war, Afghans crossing the border said they had left a frightening situation behind them.

"Most people were going toward smaller towns because the situation in the cities was bad," said 20-year-old Nazari at the Islam Qala border crossing.

As Nazari left the country, he saw Iranians looking for essential supplies.

"Especially in markets, at petrol stations, and in places that sell food, there were large crowds," he told AFP, a black backpack on his shoulders.

Iranian media have reported hundreds of casualties in the country, although AFP journalists have not been able to verify tolls independently.

At the border post in western Afghanistan, returnee Nematullah Moradi said weapons were firing "day and night".

"People were worried," the 26-year-old said, describing people evacuating from the capital Tehran where he had lived for around 18 months.

"We saw missiles landing and the smoke rising into the air afterwards," he added.

A black flag fluttered in the background as Moradi spoke, which had been raised by Iran after the killing of its supreme leader.

- 'Running in fear' -

Despite close ties and a common language, Iran has joined Pakistan in pushing back huge numbers of Afghans in recent years after hosting them for decades.

More than 1.8 million Afghans returned from Iran last year, according to the International Organization for Migration.

The UN's refugee agency said it hadn't seen a considerable uptick in people leaving Iran since the war started, but plans are in place in case numbers rise.

"Though resources are already significantly stretched amid previous high recent returns and funding shortfalls," said Charlie Goodlake, UNHCR Afghanistan spokesperson.

One Afghan trader who had gone to Iran's central city of Isfahan thought "maybe the situation would get better", but it quickly became untenable.

"When we saw that the situation was getting worse, we also started moving back" to Afghanistan, said 58-year-old Rahmatullah Sayedzada.

Israeli and US forces have kept up their bombardment of Iran, which Afghan returnees described as terrifying.

Attaullah Noori, 31, said he was packing his bags in Tehran when multiple explosions hit nearby.

"All the people -- women, men and boys -- started shouting and crying," he told AFP.

"Everyone was running in fear with their wives and children. There were people whose children were left beside the street while they ran," said Noori.

He said there were so many people fleeing the violence that "buses were not available for them to escape".

While the Afghans crossing at Islam Qala had found a way to escape the conflict, Iranians at the frontier were facing a perilous situation back home.

"I've been really worried since I heard the news about our leader's death," said an Iranian cargo driver who request anonymity for security reasons.

"I don't know what will happen in the future, but it's not a good future for us Iranians."