Ali Ahmad's eyes fill with tears as he lifts his shirt to show deep bruises across his back.

While he was detained, Iranian officers struck him and accused him of spying, he says. "They used hoses, water pipes and wooden boards to beat me. They treated us like animals."

He was speaking to the BBC earlier this month at Islam Qala on the two countries' border, before crossing back over to Afghanistan. His name has been changed to protect his identity.

Iran - which says it hosts more than four million undocumented Afghans who fled conflict in their homeland - has been stepping up deportations for months. In March those without papers were given a July deadline to depart voluntarily, but since a brief war with Israel in June, the authorities have forcibly returned hundreds of thousands of Afghans, alleging national security concerns, BBC reported.