"We have very clear evidence," the US official said, alluding to a possible video.

"It was their drone that came too close to our ship. If you fly too close to our ships, you're going to get shot down."

The Pentagon had not confirmed by midday Friday whether or not there is any photographic evidence of the downed drone.

President Donald Trump said Thursday that the USS Boxer had destroyed an Iranian drone in the Straits of Hormuz after it came dangerously close to the US warship.

Iran's armed forces dismissed Trump's assertion as "baseless and delusional" while admitting one of their drones was flying near the US ship.

"All of (Iran's) drones... have safely returned to their bases," said armed forces spokesman Brigadier General Abolfazl Shekarchi.

The Gulf region and the Strait of Hormuz, the most important shipping route for the world's oil supplies, has been a tinderbox for the past two months as Washington and Tehran face off following Trump's decision to scrap a landmark nuclear deal sign in 2015.

Trump said he came close to ordering an attack on Iranian military targets in June after Iran shot down a US drone that Tehran said had strayed into its airspace.