A fragile truce between the United States and Iran hung in the balance Saturday after US forces said they struck Iranian targets in another tit-for-tat response to attacks on shipping in the vital Strait of Hormuz. The clashes brought new tension to the negotiating process meant to end a war launched by the United States and Israel at the end of February. They also underlined the risks still facing one of the world's most important sea lanes for oil and other commodities.