The incident in the Gulf comes amid heighten tensions between the United States and Tehran.
Iranian vessels come 'dangerously' close to US military ships
Eleven vessels from Iran's Islamic Revolutionary Guard Corps Navy came close to United States Navy and Coast Guard ships in the Gulf, the US military said on Wednesday, calling the moves "dangerous and provocative".
While such interactions had occurred occasionally a few years ago, they had largely stopped in recent months and years, and this incident comes at a time of increased tensions between the two countries.
According to the statement, the Iranian ships approached six US military ships while they were conducting integration operations with Army helicopters in international waters.
At one point, the Iranian vessels came within 10 yards of the US Coast Guard cutter Maui.
The US ships issued several warnings through bridge-to-bridge radio, blasts from the ships' horns and long-range acoustic noise maker devices.
The Iranian ships left after about an hour, the statement added.
The semiofficial Fars news agency, believed to be close to the paramilitary Guard, acknowledged the incident in a report that did not include any comment from Iranian officials.
"The IRGCN's dangerous and provocative actions increased the risk of miscalculation and collision, [and] were not in accordance with the internationally recognized Convention on the International Regulations for Preventing Collisions at Sea," the US military's statement said.
On Tuesday, armed men boarded and briefly held the Hong Kong-flagged tanker SC Taipei and its Chinese crew before releasing the vessel.
The US-led International Maritime Security Construct, a group created to deter Iranian attacks in the region, acknowledged the incident in a statement late Wednesday. The group said it "assessed no immediate threat to the free flow of shipping in the area".
Tensions between the US and Iran have escalated since President Donald Trump pulled the US out of the landmark Iran nuclear deal in 2018 and re-imposed crippling sanctions.
Earlier this year, the US killed Qassem Soleimani, the head of Iran's elite Quds Force, in a drone strike in Iraq.
Iran retaliated on January 8 with a rocket attack on Iraq's Ain al-Assad base where US forces were stationed. No US troops were killed or faced immediate bodily injury, but more than 100 were later diagnosed with traumatic brain injury.