President Donald Trump has said the US Navy destroyed an Iranian drone in the Strait of Hormuz.
He said the USS Boxer amphibious assault ship "took defensive action" on Thursday after the drone came within about 1,000 yards (914m) of the vessel.
Iran said it had no information about losing a drone. In June, Iran downed a US military drone in the area.
Earlier, Tehran said it had seized a "foreign tanker" and its 12 crew on Sunday for smuggling fuel in the Gulf.
Iran has been blamed by the US for attacks on tankers which have happened in the world's key shipping area since May. Tehran denies all the accusations.
The recent incidents have triggered fears of a military conflict in the region.
What have the US and Iran said about the incident?
Speaking at the White House, Mr Trump said: "I want to apprise everyone of an incident in the Strait of Hormuz today involving USS Boxer, a navy amphibious assault ship.
"The Boxer took defensive action against an Iranian drone which had closed into a very, very near distance, approximately 1,000 yards (914m), ignoring multiple calls to stand down and was threatening the safety of the ship and the ship's crew. The drone was immediately destroyed.
"This is the latest of many provocative and hostile actions by Iran against vessels operating in international waters. The United States reserves the right to defend our personnel, facilities and interests."
A defence department official quoted by the New York Times said the drone had been brought down using electronic jamming equipment.
Meanwhile, a senior Iranian official dismissed the US statement. In an English tweet posted on 19 July, Deputy Foreign Minister Abbas Araqchi even suggested the US may have downed its own drone by mistake.