Story highlights The incident is the latest tense encounter between the United States and Iran

It occurred at the Strait of Hormuz, which is situated between the Gulf of Oman and the Persian Gulf

(CNN) A US Navy ship fired warning shots at Iranian boats on Sunday near the Strait of Hormuz, US Defense officials said Monday.

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.

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