Five Iranian vessels approached the USS Mahan and two other US ships that were entering the strait, according to accounts from four sources. The Strait of Hormuz is situated between the Gulf of Oman and the Persian Gulf.

The Mahan, a destroyer, fired warning shots and used radio calls, flares, bells and whistles to signal to the ships to stay away. There was no response to the radio calls. A US helicopter overhead dropped smoke grenades.

Pentagon spokesman Capt. Jeff Davis said the boats came "within 900 yards or so" of the Mahan.

"It's somewhat out of character, recently anyway, from what we've seen out of Iran," Davis said.

Referring to other similar encounters, Davis said there had been "a total of 35 in 2016 that were assessed to be unsafe and unprofessional," and he added that "the vast majority of those were in the first half of 2016."

"We had a significant number of these before, they had largely stopped except, you know, this incident and in August seemed to be one-offs," he continued.