Ghaus worked in Iran for five years but has nothing to show for it. All he has are memories of being jailed, beaten and sent home to Afghanistan.

Afghanistan's oil-rich western neighbour has for years been a destination for Afghans seeking work or fleeing war. Afghanistan and Iran share a language, and cultural and historical links.

But hostility to the US role in Afghanistan, regional ambitions and an economy choked by Western sanctions have persuaded Iran to cast out Afghan migrants, to the dismay of those forced home and their government.

In May, Iran threatened to expel Afghan refugees and migrant workers, in all about 2.4 million people, if Afghanistan signed a strategic security pact with the United States. The deal was struck.

"Afghan refugees and migrants are becoming the victims of big political games played between the Iranian and US powers," said Abdul Samad Hami, Afghanistan's deputy minister for refugees.

Few of the migrants, who pay smugglers about $700 to get across the 1,000 km (620 miles) border into Iran, know they have been caught up in a geopolitical power play.

Barefoot and wearing a sun-bleached silver turban, Ghaus was huddled with about 20 other expelled migrants at a UN centre in the western Afghan city of Herat after being dumped out of a bus on the border.

"Suddenly, my life came to an end," the soft-spoken bricklayer told Reuters.

Ghaus is one of 191,121 unregistered Afghan migrants forced out of Iran from January to September 30 this year, according to the UN refugee agency, up 29 per cent from last year.

About 1.4 million migrant workers remain in Iran but hundreds of them are being expelled every day. There are also nearly a million Afghan refugees who are allowed to live there.

Shuffling home through Herat's Islam Qala border checkpoint, most of the returning migrants are young men dressed like Iranians in jeans and brightly coloured bomber jackets, but with bulging suitcases and bed-rolls on their heads.

Many migrants have called Iran home for decades, part of an exodus of nine million Afghans - a third of the population - who fled to neighbours Iran and Pakistan, beginning with the Soviet invasion in 1979, through to the austere Taliban rule of the 1990s.

The expulsions represent not just a burden for Afghanistan but also a loss of income, and could even spell more instability ahead of the withdrawal of most foreign troops by the end of 2014.

Afghan migrants in Iran send home about $500 million a year, a considerable sum for one of the world's poorest countries. The funds will become more important as foreign aid shrinks with the departure of Western forces.

INFLUENCE, OPIUM

Iran says migrant workers pose a security threat. But Afghan politicians and analysts say Iran is pushing them out to show it can ratchet up the pressure when it doesn't get its way.

As the United States reduces its role in Afghanistan, neighbours are competing for influence. Iran already backs a third of Afghan media and builds schools and clinics to boost its sway, according to Afghan officials.

Iran's economic influence in Afghanistan is palpable in Herat, which enjoys robust trade over the nearby border and boasts smoothly paved roads and bustling markets.

Iran offers Afghan migrants menial work, often on construction sites. Many of the men return home addicted to opium after being paid in the drug instead of cash.

This is not only because it is cheaper for employers to pay in opium, but also because the depreciation of Iran's rial, in the face of international sanctions over its nuclear programme, has made opium more attractive than cash for the workers.

"We were allowed to choose - money or drugs. Over time, drugs seemed the better option," said Rasool, a grey-bearded man who guarded a construction site in Iran's second-largest city of Mashhad for 15 years.

Like Ghaus, Rasool was briefly imprisoned in Iran before being expelled.

Some migrants end up in prison after getting home.

"We've noticed an increase in the number of migrants coming back here with drug addictions," said General Shah Mir, who oversees Herat's provincial prison, where almost 300 former migrant workers are doing time for opium trafficking.

Health officials estimate there are 1 million Afghans addicted to opium or heroin, or about 8 per cent of the adult population, making it the world's top user per capita. It is also the top producer of opium and its refined form, heroin.

"TOUGH TALK"

But it is not just the migrant workers that Iran wants out.

It is also making moves to force out the majority of the 930,000 legal Afghan refugees and is stepping up a campaign to make them feel unwelcome, Afghan officials and rights group say.

Last month, Iran's Interior Ministry said a decision had been made to "end the status of asylum for 700,000 Afghans" by March 21, 2015.

An Interior Ministry spokesman declined to comment on the reason for the decision.

Faraz Sanei, Iran researcher at the New York-based Human Rights Watch, said Iran gets tough on repatriating Afghans when it feels pressured in the regional manoeuvring for influence.

"When political tension between Tehran and Kabul is high, for example, with regard to the US role in Afghanistan, Tehran has stepped up its tough talk," Sanei said.

Iran and Afghanistan, along with the United Nations High Commissioner for Refugees, agreed on a voluntary repatriation programme in 2002 in the belief that with better security at home, the refugees should choose to go back.

"Though this has been the official government line, we have seen an increase in the restrictive measures against Afghan refugees in recent years that has made life in Iran increasingly difficult," said Sanei.

Iran has been increasing the number of "no-go areas" for Afghans, which authorities say is part of a strategy to secure its borders. Afghans are now allowed in 12 of Iran's 31 provinces, down from 14 last year.

"Even within the allowed areas, movement of Afghans is restricted and there are only certain jobs they can do," said Hami, the deputy minister for refugees.

Afghans have even been banned from certain public parks, Iranian media has reported.

Hami said Afghanistan had been preparing for the expulsion of refugees for some time and was mapping out a "contingency plan" with the UN refugee agency that should be ready by January.

For Herat member of parliament Qazi Nazir Ahmad Hanfai, the reason Iran is putting pressure on Afghans living there is clear: Iran is punishing Afghanistan for defying its call to reject the US security pact.

"The Iranians warned the Afghans ... and now we're seeing the result of that threat," Hanfai said. – Reuters