A U.S. Navy destroyer fired warning shots Sunday after a group of Revolutionary Guard boats approached it in the Strait of Hormuz. The Iranian boats refused to change course despite radio calls, flares and smoke bombs. Reuters reports:

The officials, speaking on the condition of anonymity, said the USS Mahan established radio communication with the Islamic Revolutionary Guard Corps boats but they did not respond to requests to slow down and continued asking the Mahan questions.

The Navy destroyer fired warning flares and a U.S. Navy helicopter also dropped a smoke float before the warning shots were fired.

The Iranian boats came within 900 yards of the Mahan, but other recent encounters with Revolutionary Guard boats have been much closer. Last August there were two incidents within a week. In the first incident four Revolutionary Guard boats came within 300 yards of the USS Nitze, a guided-missile destroyer. A few days later Iranian boats came within 200 yards of the USS Squall. The Squall fired three warning shots into the water and the boats finally left the area.

There was another incident in September when Iranian boats approached the USS Firebolt. Most of the boats turned around after coming within 500 yards but one boat headed into the Firebolt’s path, forcing it to change course to avoid a collision. At the time, a defense official said there had been 31 such encounters with Iranian boats.

All of this comes after a highly publicized incident last January when Iran took 10 US sailors captive after one of their vessels broke down and drifted off course. Here’s CNN’s report on Sunday’s incident: