Iranian forces allegedly harassed US military and Coast Guard ships operating in international waters, in the second incident in the Middle East Gulf in as many days.
The US Navy's Fifth Fleet said on Wednesday that Iranian Revolutionary Guard Corps Navy (IRGCN) vessels got within 50 yards of the USS Lewis B Puller, an expeditionary sea base, and within 10 yards of the US Coast Guard ship Maui.
The incident happened in the "North Arabian Gulf", according to the navy.
"[The IRGCN] maneuvered in an unsafe & unprofessional manner violating both proper seamanship in accordance with the rules of the road, & due regard for the safety of other vessels in the area as required by international law," the Fifth Fleet tweeted.
"There were no injuries during the interaction."
On Tuesday, the 22,400-dwt SC Taipei (built 2000) was reportedly boarded by armed men and taken into Iranian territorial waters for several hours before being let go.
It was unclear whether the incident was intended to be an attack on commercial shipping similar to last year's spate of sabotage allegedly carried out by Iran.
The ship's former manager had been sanctioned by the US government for its dealings with Iran's oil industry.
That incident was preceded by two others, one on 27 March when a cargoship was approached by two skiffs in the Strait of Hormuz near the Iranian coast, and another on 2 April when a US-flag boxship was reportedly surrounded by Iranian patrol boats.
Irans economy is suffering due to US sanctions reimposed in 2018, the coronavirus outbreak that has killed more than 4,700 and low oil prices brought on in part by regional rival Saudi Arabia.
Maritime security consultancy Dryad Global said that could be the impetus for Iranian aggression in the region, although it was unlikely due to an increased military presence, with US and European missions patrolling the Middle East Gulf.
