Human Rights Watch (HRW) has called for a "thorough investigation" into "shocking" allegations that Iranian border guards beat and then forced a group of Afghan migrants into a river.
Afghan officials said up to 70 migrants last week illegally crossed into Iran, where they were beaten, tortured, and then forced into a river by Iranian border guards.
Authorities in the western province of Herat, located along the border with Iran, said they had retrieved 12 bodies from the Harirud River that crosses both countries.
Dozens of Afghan migrants are still missing and Afghan authorities have launched an operation to locate and retrieve the bodies.
"The allegations are indeed shocking," Patricia Gossman, an associate director for the Asia division at HRW, told RFE/RL on May 4. "It really requires a very thorough investigation into what exactly happened."
Gossman said that if proven, the actions of the Iranian border guards would amount to "a very serious human rights violation."
Iranian Foreign Ministry spokesman Abbas Musavi said the "incident" took place on Afghan soil.
"Border guards of the Islamic Republic of Iran denied the occurrence of any events related to this on the soil of our country," he said in a statement on May 3, adding that Tehran would launch an investigation into the incident.
Abdul Ghani Noori, governor of Herat's Gulran district, where the migrants crossed, accused Iranian security forces of throwing the group of Afghan migrants into the Harirud River.
Afghan officials said it was not the first time that Afghans had been tortured and killed by Iranian security forces guarding the 920-kilometer-long border.
"I haven't heard of a case like this in recent memory, although we have previously documented abuses by Iranian border officials against Afghans for some time," Gossman said, adding that there had been past incidents of Iranian border guards beating and firing on Afghan migrants.
Decades of conflict, extreme poverty, and high rates of unemployment force thousands of Afghans to illegally cross the border into Iran every year.
There are currently up to 1 million registered Afghan refugees in Iran, while the country hosts another 2 million undocumented Afghans, according to the United Nations.