Iranian Navy speed boats harassed US naval vessels in two recent incidents in the Strait of Hormuz, a senior US defense official said, confirming a CNN report.
The first incident occurred as the USS New Orleans, an amphibious transport ship, was sailing last week through the Strait of Hormuz into the Persian Gulf.
Three Iranian Navy speed boats rapidly approached within 500 yards of the US ship, according to US officials cited by CNN.
The second incident involved a US Coast Guard cutter off the Kuwaiti coast, similarly approached by an Iranian speedboat.
Sailors aboard the cutter USCGC Adak reported seeing Iranians aboard the speed boat brandishing AK-47 assault rifles and a heavy machine gun, CNN said.
"I can confirm there was some harassment," a senior official told AFP.
US-Iranian tensions have worsened in recent weeks amid Western moves to tighten sanctions on the Iranian regime over its nuclear program and a series of incidents.
These include the assassination of an Iranian nuclear scientist, a death sentenced handed down against an American-Iranian former Marine accused of spying for the CIA, and the Iran's capture last month of what it said was a CIA drone.
Iran has responded to western threats of an oil embargo by warning it will close the Strait of Hormuz -- a chokepoint for 20 percent of the world's oil at the entrance to the Gulf -- in retaliation.