A top military official in Iran warned the U.S. and Israel on Monday of a “devastating” response after he blamed the two countries of playing a role in Saturday's assault that killed 25, including 12 members of the Revolutionary Guards, Reuters reported.

Militants disguised as soldiers opened fire on an annual Iranian military parade in the southwestern city of Ahvaz. The shooting was the deadliest terror attack to strike the country in nearly a decade.

Hossein Salami, the deputy head of the Revolutionary Guard, was at a funeral for one of the attack’s victims when he reportedly said, “You have seen our revenge before ... You will see that our response will be crushing and devastating and you will regret what you have done.”

Suspects have reportedly been arrested and the country has declared Monday a national day of mourning. Iran believes the attack was carried out by militants trained by Israel and Gulf states, and supported by the U.S.

The U.S. 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."

“We stand with the Iranian people against the scourge of radical Islamic terrorism and express our sympathy to them at this terrible time,” said Heather Nauert, the U.S. State Department spokeswoman.

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 Defense Week."

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

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

"Oh God! Go, go, go! Lie down! Lie down!" one man screamed as a woman fled with her baby.

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

The region's Arab separatists, once only known for nighttime attacks on unguarded oil pipelines, claimed responsibility. Reuters reported that Islamic State’s Amaq agency posted a video that purportedly showed three men in a vehicle on their way to the parade.

“We are Muslims, they are kafirs (non-believers),” the man reportedly could be heard saying. He went on: “We will destroy them with a strong and guerrilla-style attack, inshallah (God willing).”

Tensions have been on the rise between Iran and the U.S. The Trump administration in May pulled out of the 2015 nuclear accord with Iran, and since then has 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.

Iran summoned diplomats from Britain, Denmark and the Netherlands early Sunday for allegedly harboring "members of the terrorist group" that launched the attack.

State TV hours later reported that all four gunmen had been killed, with three dying during the attack and one later succumbing to his wounds at a hospital.

The Associated Press contributed to this report