"The United States has committed a new error while carrying out this senseless and savage act," Ahmadinejad was quoted as telling Syrian President Bashar al-Assad during a telephone conversation.
"The Iranian people and government will always be on the side of their Syrian brothers," he added.
Iran is Syria's chief regional ally.
Damascus says eight civilians, including children, were killed in a helicopter assault on Sunday launched by US troops from Iraq on a Syrian village near the Iraqi border.
The operation has provoked condemnation from a number of world governments.
A US official in Washington has said the operation targeted a top militant who smuggled arms and fighters into Iraq but officially the State Department and Pentagon have declined to comment.
On Tuesday, Syria protested to the UN Security Council over what it branded a barbaric action, with the official press calling it a "cold-blooded war crime."
US commanders say Syria is the main transit point for foreign jihadists crossing into Iraq and have blamed Damascus for turning a blind eye to the problem.
