Iranian naval vessels veered close to American warships this week in a series of incidents that American officials described as harassing maneuvers risking dangerous escalation, defense officials said Thursday.

The first incident occurred Tuesday, when Iranian ships made provocative maneuvers around a U.S. destroyer in the Strait of Hormuz, officials said. The following day, Iranian vessels came within several hundred meters of other American ships in the Persian Gulf, with one Iranian ship prompting the coastal patrol ship USS Squall to fire warning shots.

“These were incidents that the crews deemed unsafe,” Pentagon spokesman Peter Cook told reporters. “These are incidents that carry a risk of escalation, and we don’t desire any kind of escalation. Our ships have been operating in that part of the world for years.”

William Urban, a spokesman for the 5th Fleet, said four vessels from Iran’s Revolutionary Guard Corps (IRGC) conducted a “high-speed intercept” in the first incident Tuesday, passing close to the USS Nitze, an Arleigh Burke-class destroyer, as it transited international waters.

The Iranian ships that buzzed the Nitze ignored repeated radio, whistle and flare warnings from the Nitze and slowed their approach only when they were within 300 yards of the U.S. ship, Urban said. He described the Iranian actions as “unsafe and unprofessional.”

The Nitze was accompanied by the USS Mason, a guided-missile destroyer, as it made what Urban described as a “routine transit” through an area that is an important international maritime thoroughfare.

“The Iranian high rate of closure on a United States ship operating in accordance with international law while transiting in international waters along with the disregard of multiple warning attempts created a dangerous, harassing situation that could have led to further escalation including additional defensive measures by Nitze,” Urban said in a statement.

On Wednesday, three IRGC ships approached the Squall and the USS Tempest, another a coastal patrol ship, at high speed in the northern Persian Gulf, Urban said. Later in the day, an Iranian vessel came within 200 yards of the Tempest. After the Tempest shot flares and tried to communicate using the ship’s loudspeaker, Squall personnel fired three shots into the water from that ship’s .50-caliber gun. The Iranian ship then departed.

That same Iranian ship later approached the USS Stout, a guided-missile destroyer, later Wednesday. “The [IRGC] vessel proceeded to cross the bow of the Stout at close range on three separate occasions,” Urban said.

The incidents are the latest sign of U.S.-Iranian friction near Iran’s coast. In January, a group of U.S. sailors was detained overnight by Iranian personnel after their boats veered into Iranian waters. It was an embarrassment for the Obama administration and resulted in disciplinary action for some of the U.S. personnel involved.

Long-standing U.S. tensions with Iran over Tehran’s support for militant groups across the Middle East have not dissipated in the wake of a 2015 deal to curb Iran’s nuclear program. The countries are backing opposite sides in the conflict in Syria and remain at arm’s length in Iraq despite shared support for the government in Baghdad.

Read more: