IRAN has summoned diplomats from Britain, Denmark and the Netherlands over allegedly harbouring members of a terrorist group that launched an attack on a military parade in the country’s south-west.

The country’s Foreign Ministry also criticised Britain over a Saudi-linked satellite channel immediately airing an interview with an Ahvazi separatist claiming responsibility.

Saturday’s attack in Ahvaz killed at least 25 people and wounded over 60 in the deadliest terror atrocity to strike the country in nearly a decade.

The attack came as rows of Revolutionary Guardsmen marched down Ahvaz’s Quds, or Jerusalem, Boulevard.

It was one of many around the country marking the start of Iran’s long 1980s war with Iraq, commemorations known as the “Sacred Defence Week”.

Onlookers turned to look toward the first shots, then rows of marchers broke as soldiers and civilians sought cover under sustained gunfire.

Women and children scattered along with the Revolutionary Guard soldiers as heavy gunfire rang out and the chaos was captured live on state television.

Iranian soldiers used their bodies to shield civilians in the melee, with one Guardsman in full dress uniform and sash carrying away a bloodied boy.

In the aftermath, paramedics tended to the wounded as soldiers, some bloodied, helped their comrades to ambulances.

Iran has blamed its Mideast arch-rival, the Sunni kingdom of Saudi Arabia, for funding the Arab separatists’ activity.

State media in Saudi Arabia did not immediately acknowledge the attack, though a Saudi-linked, Farsi-language channel based in the United Kingdom immediately carried out an interview with an Ahvazi activist claiming responsibility.

The region’s Arab separatists, once only known for nighttime attacks on unguarded oil pipelines, claimed responsibility for the assault and Iranian officials believed the claim.

Hamid Baeidinejad, Iran’s ambassador to the UK, called it “heinous act” on Twitter and added his country would file a complaint with British authorities.

Following the attack, a local news source said at least eight of the people killed had served in the Revolutionary Guard, an elite paramilitary unit that answers only to Iran’s supreme leader.

State TV reported the four gunmen had been killed, with three dying in the attack and one succumbing to his wounds in hospital.

President Hassan Rouhani ordered Iran’s Intelligence Ministry to immediately investigate the attack.

Meanwhile, supreme leader Ayatollah Ali Khamenei described the attack as exposing “the atrocity and viciousness of the enemies of the Iranian nation”.

Iranian foreign minister Mohammad Javad Zarif blamed regional countries and their “US masters” for funding separatists, issuing a warning as tensions remain high after the US’ withdrawal from the Iranian nuclear deal.

Tensions have been on the rise between Iran and the US since Donald Trump’s administration pulled out of the 2015 nuclear accord with Iran in May, and re-imposed sanctions that were eased under the deal.

It also has steadily ramped up pressure on Iran to try to get it to stop what Washington calls “malign activities” in the region.

Despite that, the US government strongly condemned the attack and expressed its sympathy, saying that “the United States condemns all acts of terrorism and the loss of any innocent lives”.

Saturday’s attack comes after last year’s Daesh assault on parliament and the shrine of Ayatollah Ruhollah Khomeini, the leader of Iran’s 1979 Islamic Revolution. At least 18 people were killed and more than 50 wounded.

That assault shocked Tehran, which largely has avoided militant attacks in the decades after the tumult surrounding the revolution.

In the last decade, mass-casualty militant attacks have been rare.