In a report to the U.S. Congress seen before its Tuesday release by The Wall Street Journal, the Pentagon noted U.S. troop deaths declined to 37 last month and 13 this month from 126 in May, with a similar decline in Iraqi fatalities.

Washington has alleged for months that Tehran provided training for insurgents, along with Iranian-built armor-penetrating roadside bombs.

A U.S. military official in Baghdad who asked not to be identified agreed with the report.

"At times we think we are seeing positive signs, then all of a sudden we capture some people who were just recently trained in Iran," the official said. "We have seen some indications of lowered attacks using the weapons, but we are not sure if it is due to their stopping the flow, our finding them and the networks, or a combination of both or neither."