Four Iranian boats made a high-speed approach at the destroyer USS Mahan on Sunday, obliging the Navy vessel to fire warning shots after they ignored radio requests to break off.

USNI News reports the Iranian boats came within 900 yards of the Mahan, which was escorting two U.S. ships through the Strait of Hormuz, the amphibious assault ship USS Makin Island and a fleet oiler.

The Iranian group consisted of four fast inshore attack craft, approaching at a high rate of speed with their crew-served weapons manned, according to a U.S. official.

“After several attempts to warn off the boats with radio communications, siren and the ship’s whistle the boats came within 900 yards of the guided missile destroyer before the crew fired three warning shots from one of the ships .50 caliber,” USNI News writes. “A helicopter from Makin Island also deployed a smoke screen generator, a so-called “smoke float” that did not deter the IRGCN boats.”

The UK Guardian clarifies that radio communication with the Iranians was established, but they ignored requests to slow down and “continued asking the Mahan questions.”

The U.S. Navy described the Iranians’ behavior as “unsafe and unprofessional.” The U.S. official quoted by the Guardian said the Mahan had a total of seven “interactions” with vessels from Iran over the weekend, but the other six were “judged to be safe.”

As with several previous tense encounters, the boats were under the command of the Iranian Revolutionary Guard Corps Navy (IRGCN), which is separate from the Iranian Navy, reports directly to the theocratic government, and is given great discretion to “boldly and courageously” defend the Iranian coastline.