Ten Iranian border guards were killed by Sunni militants in a cross-border attack on the frontier with Pakistan on April 26, Iranian media reported.
The militant group called Jaish al Adl, or the Army of Justice, has claimed responsibility for the attack in Sistan-Baluchestan province, reports said.
Iranian police said the guards were killed by long-range guns and "the Pakistani government bears the ultimate responsibility for the attack."
Sistan-Baluchestan province in southeastern Iran has long been plagued by unrest from both drug smuggling gangs and separatist militants. The population of the province is predominantly Sunni Muslim while the majority of Iranians are Shi'ites.
Jaish al Adl is a Sunni militant group that has carried out several attacks before against Iranian security forces to highlight what it claims is discrimination against Sunni Muslims and the ethnic Baluch in the province.
The group claimed responsibility for attacks that killed eight border guards in April 2015 and 14 border guards in October 2013.