Iranian President Hassan Rouhani on Sunday pointed blame at Arab separatists for a deadly attack on a military parade and accused an unnamed United States-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 near the Iraqi border that left at least 29 people dead.

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 militant Islamic State (IS) group claimed responsibility for the rare assault.

But from early on, Iranian officials saw an Arab separatist movement, the Ahwazi Democratic Popular Front (ADPF) or Al-Ahwazi, as the main suspect.

“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 on Sunday, shortly before leaving Tehran for the United Nations 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.”

“All these little mercenary countries we see in this region are backed by America. It is the Americans who incite them,” he said.

Iranian Foreign Minister Mohammad Javad Zarif said the attack had been carried out by “terrorists recruited, trained, armed and paid by a foreign regime”.

London-based opposition channel Iran International TV on Saturday 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”.

But in a statement on its website, the group denied any involvement, accusing Iranian authorities of ordering the attack to distract from Tehran's support for “militias in the region”.

Diplomats summoned

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 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 responsibility for this terrorist act through a London-based TV network,” said the ministry's spokesman, Bahram Ghasemi.

The British foreign ministry said its charge d'affaires had extended its condolences to Tehran and that Iranian officials were planning to lodge a formal complaint with Britain's media watchdog Ofcom.

Ghasemi also said Iran expected the Danish and Dutch governments to “hand over the perpetrators of this attack and anyone related to them to Iran for a fair trial”.

He also said Iran had warned the United Arab Emirates over “offensive remarks” attributed to a UAE “political advisor” following the attack.

“The charge d'affaires was warned that blatant support of terrorist acts by those linked to Emirati authorities will have repercussions for the UAE government,” Ghasemi said, without disclosing the official's identity or the nature of the comments.

State television gave a toll of 29 dead and 57 wounded in the attack, while official news agency IRNA said those killed included women and children who were spectators at the parade.

Three attackers were killed at the scene and the fourth died later of his injuries, said armed forces spokesman Brigadier General Abolfazl Shekarchi.

IS had claimed the attack via its propaganda mouthpiece Amaq and that the attack was in response to Iranian involvement in conflicts across the region.

The Revolutionary Guards accused Iran's arch-rival Saudi Arabia of funding the attackers, while Iran's supreme leader Ayatollah Ali Khamenei also blamed Iran's pro-US rivals.

Khuzestan, which has a large ethnic Sunni Arab community, was a major battleground of the 1980s war with Iraq and it saw unrest in 2005 and 2011, but has since been largely quiet.

Kurdish rebels frequently attack military patrols on the border further north, but attacks on government targets in major cities are rare.

On June 7, 2017 in Tehran, 17 people were killed and dozens wounded in simultaneous attacks on the parliament and on the tomb of revolutionary leader Ayatollah Ruhollah Khomeini — the first inside Iran claimed by IS.