The US Navy on Wednesday accused Iran's Islamic Revolutionary Guard Corps of harassing American ships in the Gulf amid heightened tension between Tehran and Washington.
The US Fifth Fleet tweeted that 11 Iranian vessels “repeatedly conducted dangerous and harassing approaches" towards US Navy ships in the north Arabian Gulf.
It also released a video documenting the incident.
It said that the US crew took action "deemed appropriate to avoid collision”.
The US Navy said there were no casualties but one vessel came to within 10 metres of its ship.
The US Navy said the Republican Guard ships repeatedly crossed the bows and sterns of the US vessels at very close range and high speeds.
Secretary of State Mike Pompeo told the Fox News channel that the US was evaluating its response to the incident.
Asked whether he had discussed the incident with the Pentagon, Mr Pompeo said: "We've talked as a team. ... We're evaluating how best to respond and how best to communicate our displeasure with what ... took place."
The Revolutionary Guard was blamed after a Hong Kong-flagged ship was boarded in the Gulf of Oman on Tuesday.
The UK Maritime Trade Organisation said later on Tuesday that the vessel had been released and was “under control of the master”, with all of its crew safe.
The maritime organisation said the vessel "had been boarded by armed men while at anchor" near the strait off the coast of Iran.
Last year, Iran was accused by the US of launching attacks against several oil tankers.
Iran seized at least one British-flagged tanker and approached another.
Iran also shot down a US drone last June, and its leaders have called on US navy to leave the region.
But the US administration of President Donald Trump sent more ships to the area, and threatened Iran with a harsh response in an event of an attack.
In response to the ship attacks, the International Maritime Security Construct, based in Bahrain, was launched in November.
It is designed to protect navigation in the Gulf, including the Strait of Hormuz.
Manama, Bahrain's capital, is also where the US Fifth Fleet is based.
Washington and other western powers have accused Tehran of lashing out after the US withdrew from the nuclear deal signed with world powers in 2015.
It imposed crippling sanctions on the Iranian economy to disrupt its activity across the Middle East.
Earlier on Wednesday, Iranian Foreign Minister Javad Zarif said the world was learning that the US "kills people".
Mr Zarif's comments came after Mr Trump suspended US funding for the World Health Organisation amid the coronavirus pandemic.
