In a speech to US war veterans in Reno, Nevada, Mr Bush renewed charges that Tehran has provided training and weapons for extremists in Iraq.
"The Iranian regime must halt these actions," he said.
In his speech to the American Legion, Mr Bush hit back, accusing Iran's Revolutionary Guards of funding and arming insurgents in Iraq.
And he said Iran's leaders could not avoid some responsibility for attacks on coalition troops and Iraqi civilians.
"I have authorised our military commanders in Iraq to confront Tehran's murderous activities," he said.