"At 4 pm (1130 GMT) on Friday, the supercarrier USS Nimitz and its accompanying warship, while being monitored by the Guards' frigates, flew a helicopter near the Resalat oil and gas platform and approached the force's ships," the Guards said.
"The Americans in a provocative and unprofessional move, sent a warning message to the frigates and fired flares."
The Guards "ignored the unconventional move by the US ships and continued their mission, after which the supercarrier and its warship left the area," the statement added.
On Tuesday, a US Navy patrol ship fired warning shots at a Guards boat in the Gulf as it closed in on the American vessel, US officials said.
The Guards denied approaching the US ship on that occasion and said it was the American vessel that had been at fault.
There have been a string of close encounters between US ships and Iranian vessels in the Gulf in recent months.
In January, the USS Mahan destroyer fired warning shots at four Iranian Revolutionary Guard vessels that approached at high speed in the Strait of Hormuz, which connects it to the Indian Ocean.
The Revolutionary Guards are a paramilitary force that answers directly to Iran's supreme leader Ayatollah Ali Khamenei.
The force's boats periodically approach US warships in international waters and the Strait of Hormuz, ignoring US radio messages and giving little indication of their intentions.
In January 2016, the Iranians briefly captured the crew of two small US patrol boats that strayed into Iranian waters.
The 10 US sailors were released 24 hours later.
The latest incident comes after the US Congress approved new sanctions against Iran.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)