DUBAI, Sept 23 (Reuters) -

Iran's Revolutionary Guards vowed on Sunday to wreak "deadly and unforgettable" vengeance for a shooting attack on a military parade that killed 25 people, including 12 of their comrades, and Tehran accused Gulf Arab states of backing the gunmen.

Saturday's assault, one of the worst ever against the elite force of the Islamic Republic, struck a blow at its security establishment at a time when the United States and its Gulf allies are working to isolate Tehran.

"Considering (the Guards') full knowledge about the centers of deployment of the criminal terrorists' leaders..., they will face a deadly and unforgettable vengeance in the near future," the Guards said in a statement carried by state media.

The assailants fired on a viewing stand in the southwestern city of Ahvaz where Iranian officials had gathered to watch an annual event marking the start of the Islamic Republic's 1980-88 war with Iraq. Soldiers crawled about as gunfire crackled. Women and children fled for their lives.

There has been a blizzard of furious statements from top Iranian officials since the attack directed at the United States and Gulf kingdoms, blaming them for the bloodshed and threatening a tough response.

"The Persian Gulf states are providing monetary, military and political support for these groups," President Hassan Rouhani said before leaving Tehran to attend a U.N. General Assembly meeting in New York.

The attack on the military parade is likely to give security hardliners like the Guards more political ammunition because they did not endorse the pragmatist Rouhani's pursuit of the nuclear deal with the West, analysts say.