Iranian Foreign Minister Mohammad Javad Zarif blamed "a foreign regime" backed by the United States for Saturday's attack on a military parade which killed at least eight troops and several civilians.
"Terrorists recruited, trained, armed & paid by a foreign regime have attacked Ahvaz," Zarif said in a tweet, adding: "Iran holds regional terror sponsors and their US masters accountable for such attacks.
"Iran will respond swiftly and decisively in defence of Iranian lives," Zarif vowed, adding "children and journos" were "among casualties".
The gunmen opened fire on the large crowd of spectators watching the parade in the southwestern city of Ahvaz and then attempted to attack the viewing stand for official dignitaries before being shot and wounded by security forces, the semi-official Fars news agency said.
The deputy governor of Khuzestan province, Ali-Hossein Hosseinzadeh Hosseinzadeh, told the semi-official news agency ISNA that "eight to nine" troops were killed and more than "20 injured".
The rare attack targeted Khuzestan, a province bordering Iraq that has a large ethnic Arab community and has seen separatist violence in the past which Iran has blamed on its regional rivals.