Saying the separatists who killed at least 25 people at an Iranian military parade are allied with the US and Saudi Arabia, Ayatollah Ali Khamenei says those responsible "will certainly be severely punished".
Gunmen fired on a viewing stand in Ahvaz where officials had gathered to watch an annual parade marking the start of Iran's 1980-88 war with Iraq. Many held pictures of a four-year-old boy killed in the incident, one of the worst such attacks against Iran's most powerful military force.
Of the 25 people killed at Saturday's parade, 12 were from Ahvaz and the rest from elsewhere in Khuzestan province.
Fars and IRNA news agencies said on Monday five attackers were killed, not four as previously reported by state media.
Iran has declared Monday a national day of mourning. The shooting was the deadliest terror attack to strike the country in almost a decade.
Explaining why Iranian athletes do not take part in any match with the representatives of the Zionist regime of Israel, Ayatollah Khamenei said, "After the victory of the Islamic Revolution, the Islamic Republic did not recognize the Zionist regime and the [former] racist regime of South Africa".
"We stand with the Iranian people against the scourge of radical Islamic terrorism and express our sympathy to them at this awful time", said Heather Nauert, the U.S. State Department spokeswoman.
At Monday's ceremony, Revolutionary Guard's acting commander Gen. Hossein Salami vowed revenge against the attack's perpetrators and what he called the "triangle" of Saudi Arabia, Israel and the U.S. Arab separatists have claimed the assault, which also wounded 60, including Guard members and soldiers.More news: Jack Ma makes a grim prediction about US-China trade war
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The deputy head of the Revolutionary Guards accused Saudi Arabia, Israel and the USA of masterminding the attack and promised they would face repercussions.
There has been a blizzard of furious statements from top Iranian officials, including President Hassan Rouhani, accusing Iran's adversaries the United States and Gulf states of provoking the bloodshed and threatening a tough response.
Two groups have claimed responsibility for the attack: the ethnic Arab antigovernment Ahvaz National Resistance and the extremist group Islamic State (IS).
The United States was not the only country to delay its messages of condolences to Iran - notably, unlike many other nations, it does not have an ambassador in Tehran.
It is, however, highly unlikely the Guards will strike any of its foes directly and risk setting off a regional conflict. "It is Americans who instigate them and provide them with necessary means to commit these crimes", Rouhani said before leaving for the U.N. General Assembly in NY.
"Iran is not the place for such [terrorist] activities", Alav said.
The accusation will nearly certainly antagonize Iran's regional foe Saudi Arabia.
It answers to Supreme Leader Ayatollah Ali Khamenei. The oil super-powers are waging a war for influence across the Middle East, backing opposite sides in Syria, Yemen, Iraq and Lebanon.