At least 10 members of Iran's Revolutionary Guards were killed when militants attacked one of their bases along the border with Iraq, the force said in a statement on Saturday.
The attack happened on Friday night in the village of Dari, in the Marivan district of Iran's northwestern Kurdistan region, some 620 kilometers (385 miles) west of the capital Tehran.
"The attack by the evil rebels and terrorists against a revolutionary border post and the explosion of a munitions depot caused the martyrdom of 10 fighters," the Guards' ground forces division said in the statement.
There was some confusion over the announcement, as the statement listed 11 names of "martyred" soldiers.
It added that "several terrorists" had also been killed and injured in the clash.
"A merciless vengeance awaits them," the statement added.
Provincial security official Hosein Khosheqbal told state television that 11 members of the Guards' voluntary Basij forces were killed in the overnight violence, which he blamed on the PKK terrorist group's Iranian offshoot the Party of Free Life of Kurdistan (PJAK).
"The latest news is that the Basij and Guards forces are in hot pursuit of the attackers," Khosheqbal said.
It was the largest number of Iranian troops killed in a single attack on the Iraqi border in recent years, though the area has seen occasional fighting between Iranian forces and PJAK terrorists as well as militants linked to the Daesh terrorist group.
Earlier this month, the Revolutionary Guards said they had killed three militants in a security operation near the border with Iraq, and nine militants were reported killed by the Guards last month further north on the border.
There is little coordination between Iranian and Iraqi forces over security of the porous border.
Intelligence Minister Mahmoud Alavi said on Tuesday that security forces in southwest Iran arrested four suspected Daesh operatives who were planning attacks.
In June 2017, Daesh militants carried out coordinated attacks at the parliament building in Tehran and the mausoleum of Iranian revolutionary leader Ayatollah Ruhollah Khomeini south of the capital, killing at least 18 people.