Seven Iranian military boats harassed a US patrol ship in international waters in the Persian Gulf over the weekend in yet another "unsafe and unprofessional" interaction, the Pentagon said Tuesday.

The Iranian Revolutionary Guard Corps fast-attack boats approached the USS Firebolt on Sunday with their machine guns uncovered, though not trained on the Americans, according to Pentagon spokesman Captain Jeff Davis.

"Three of them maneuvered close to the (US) ship, shadowing her course from a range of about 500 yards (meters)," Davis said.

The Iranian vessels eventually broke away, but one then turned towards the Firebolt and stopped directly in front of it.

"This caused the Firebolt to have to maneuver to avoid collision," Davis said, noting that the US sailors had attempted to hail the Iranians via radio.

"This is another example of an unsafe and unprofessional interaction we've had with the IRGCN."

Sunday's encounter is at least the fifth the Pentagon has revealed in the past month, with US military officials repeatedly blasting Tehran for the maritime incidents.

In one incident last month, the USS Squall resorted to firing warning shots from a 50-caliber gun at an Iranian vessel as it approached.

Navy officials say ships from the US and Iranian navies interacted more than 300 times in 2015 and more than 250 times the first half of this year, with 10 percent of those encounters deemed unsafe and unprofessional.

"Commanding officers of ships are given the inherent right of self-defense, and what we don't want to have is a situation where, due to miscalculation, that right is invoked when it doesn't need to be," Davis said.

In January, the Iranian navy briefly captured the crews of two US patrol boats that had, through a series of blunders, strayed into Iranian territorial waters.

The 10 American sailors were released within 24 hours.