Iranian President Hassan Rouhani on Sunday pointed blame at Arab separatists for a deadly attack on a military parade and accused an unnamed US-backed Gulf state of supporting them.
Tehran also summoned diplomats from Denmark, the Netherlands and Britain for allegedly hosting members of the group suspected of links to Saturday's attack that killed at least 29 people.
Four militants attacked a parade commemorating the start of the 1980-1988 Iran-Iraq war in the southwestern city of Ahvaz, capital of the border province of Khuzestan.
Officials and an eyewitness said the gunmen were clad in Iranian military uniforms and had sprayed the crowd with gunfire using weapons they had stashed in a nearby park.
The Islamic State (IS) jihadist group claimed responsibility for the rare assault.
But Iranian authorities see an Arab separatist movement, the Ahwazi Democratic Popular Front (ADPF) or Al-Ahwazi, as the main suspect. "One of the countries in the south of the Persian Gulf took care of their financial, weaponry and political needs," said President Rouhani.
"All these little mercenary countries we see in this region are backed by America. It is the Americans who incite them."