The Pentagon said the Iranian Revolutionary Guard Corps boats "repeatedly crossed the bows and sterns of the US vessels at extremely close range and high speeds," coming at one point within 10 yards (nine meters) of the bow of one.

At the time the US Navy and Coast Guard ships were conducting operations in international waters in the northern Gulf with Apache attack helicopters, the US Navy said in a statement.

The much smaller speedboat-like IRGC Navy vessels ignored warnings from the US ships for about one hour, before finally responding to radio communications and then leaving, the US side said.

"The IRGCN's dangerous and provocative actions increased the risk of miscalculation and collision," the US said, adding they violated maritime customs and laws.