The Coast Guard cutter Monomoy sent a small, inflatable boat to approach the Iranian dhow and when it got close, the Americans spotted two 50-caliber machine gun, with one of the guns aimed at them, US military officers said.
"They saw that one of the machine guns was manned and trained on them," Lieutenant Joseph Hontz, a spokesman for the Fifth Fleet, told AFP.
The Americans pulled back and fired one warning shot, he said, while the Iranian vessel headed off.
"In any event, the dhow pulled away and nobody was hurt," Pentagon spokesman Rear Admiral John Kirby told reporters.
He said the Coast Guard vessel was on a "routine maritime security operation."