Iranian President Hassan Rouhani on Sunday appeared to blame Arab separatists for an attack on a military parade the previous day that left 29 people dead.
"It is absolutely clear to us who has done this, which group it is and to whom they are affiliated," Rouhani said on state television shortly before leaving Tehran for the UN General Assembly in New York.
"Those who have caused this catastrophe ... were Saddam's mercenaries as long as he was alive and then changed masters," he said, referring to late Iraqi dictator Saddam Hussein.
"One of the countries in the south of the Persian Gulf took care of their financial, weaponry and political needs," Rouhani added.
"All these little mercenary countries we see in this region are backed by America. It is the Americans who incite them", he said.
Four militants on Saturday attacked the parade commemorating the beginning of the 1980-88 Iran-Iraq war, launched by Baghdad, in the southwestern city of Ahvaz, capital of Khuzestan Province.
Officials and an eyewitness said the gunmen were dressed in Iranian military uniforms and 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 from the start Iranian officials saw an Arab separatist movement, the Ahwazi Democratic Popular Front (ADPF), also known as Al-Ahwazi, as the main suspect.
On Saturday, the London-based Iran International TV aired an interview with Yaqoub Hor Altostari, presented as a spokesman for ADPF, indirectly claiming responsibility for the attack and calling it "resistance against legitimate targets".
Iran in response summoned diplomats from Denmark, the Netherlands and Britain to complain about them "hosting some members of the terrorist group" and "double standards in fighting terrorism," the foreign affairs ministry said.
The British charge d'affaires "was told that it is not acceptable that the spokesman for the mercenary Al-Ahwazi group be allowed to claim responsiblity for this terrorist act through a London-based TV network," said the ministry's spokesman, Bahram Ghasemi.
"It is expected that (the Danish and Dutch) governments hand over the perpetrators of this attack and anyone related to them to Iran for a fair trial," he added.