At a news conference, U.S. Marine Gen. Peter Pace, chairman of the Joint Chiefs of Staff, offered few details other than saying markings on the plastic explosive indicated Iranian manufacture, The New York Times reported.
"We have intercepted weapons in Afghanistan headed for the Taliban that were made in Iran," Pace said but he refused to speculate if the Iranian government was aware of weaponry movement.
"Either the leadership of the country knows what their armed forces are doing or they don't know. And in either case that's a problem," the general said.
Pace gave few other details about the seizure but another military official later said it was conducted in the last week or so in the southern city of Kandahar, the Times said.
U.S. commanders in Iraq have claimed in recent months that Iran is supplying munitions and training to Iraqi insurgents. Tehran denies any such involvement.