“Terrorists recruited, trained, armed, and paid by a foreign regime have attacked Ahvaz. Children and journos [are] among casualties,” Zarif tweeted on Saturday, presstv.com reported.
Iran holds “regional terror sponsors” and their “US masters” responsible for the attack, he said.
At least 10 were killed when gunmen opened fire on people from behind a viewing stand at Qods Boulevard of Ahvaz during the morning parade held to mark the former Iraqi dictator Saddam Hussein's invasion of Iran in the 1980s.
"Individuals disguised in the Islamic Revolution Guards Corps and Basij uniforms fired at officials and people from behind the stand, leaving a number of innocent people including women and children martyred or injured," Governor of Khuzestan Province Gholamreza Shariati said.
"So far, the martyrdom of three people has been verified in this subversive and anti-security action, with more than 20 other injured who were transferred to hospital," he told the official IRNA news agency.
The Islamic Revolution Guards Corps (IRGC) spokesman Ramezan Sharif said the attackers were affiliated to a terrorist group supported by Saudi Arabia.
Earlier in September, the IRGC confirmed that it had fired seven short-range missiles at a gathering of terrorist commanders in Iraq's semi-autonomous Kurdistan region.
The Corps said the strike had come following "wicked actions" in the previous months by terrorist groups affiliated with the global arrogance against Iran's border areas.