Iranian Supreme leader Ayatollah Ali Khamenei accused U.S.-backed Gulf Arab states of carrying out an attack on a military parade that killed 25 people, almost half of them members of the country's elite Revolutionary Guards.
Khamenei ordered security forces to bring to justice the "criminals" behind one of the worst attacks ever against the Revolutionary Guards, who answer to him.
The allegation will almost certainly ratchet up tensions with Iran's regional rival Saudi Arabia and its Gulf allies, who along with the United States have been working to isolate the Islamic Republic.
"This crime is a continuation of the plots of the regional states that are puppets of the United States, and their goal is to create insecurity in our dear country," Khamenei said in a statement published on his website.
He did not name the regional states he believed were to blame. Israel is also a key U.S. ally opposed to Tehran.
State television said the assault, which wounded more than 60 people, targeted a stand where Iranian officials had gathered in the city of Ahvaz to watch an annual event marking the start of the Islamic Republic's 1980-88 war with Iraq.
An Iranian ethnic Arab opposition movement called the Ahvaz National Resistance, which seeks a separate state in oil-rich Khuzestan province, claimed responsibility for the attack.
Islamic State militants also claimed responsibility. Neither claim provided evidence. All four attackers were killed.
Women and children also died in the assault, state news IRNA agency reported.
Brigadier General Abolfazl Shekarchi, a senior spokesman for Iran's armed forces, said the attackers had hidden weapons in an area near the parade route several days in advance.
"All four terrorists were quickly neutralised by security forces," Shekarchi told state television. "A four-year-old girl and a wheelchair-bound war veteran were among the dead."
Attacks on the military are rare in Iran.
The bloodshed struck a blow to security in OPEC oil producer Iran, which has been relatively stable compared with neighbouring Arab countries that have grappled with upheaval since the 2011 uprisings across the Middle East.

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