The US military on Wednesday released a video showing Iranian troops boarding a ship in the Arabian Gulf.
US Central Command posted footage on Twitter, which it said showed the moment two Iranian ships and a helicopter overtook the ship.
"Today in international waters, Iranian forces, including two ships and an Iranian Sea King helicopter, overtook and boarded a ship called the Wila," it said.
The video, in English and Farsi, shows a helicopter approaching the ship and landing soldiers on it.
The Iranian government and official media did not immediately comment on the incident. A US defence official told Agence France-Presse on Thursday that the Iranians released the vessel after holding it for four to five hours.
Centcom is calling on Iran to explain its aggressive behaviour.
Army Maj John Rigsbee, a Centcom spokesperson, told The National that Iran's actions violate international law and undermine freedom of navigation and commerce.
"We call on Iran to explain its actions to the international community and articulate its legal basis for its actions," Maj Rigsbee said.
"This type of reckless, aggressive behaviour by Iran destabilises the region and threatens the rules based international order."
The incident occurred in international waters of the Gulf of Oman, 32 kilometres off the coast of the UAE.
"A coalition ship monitored the event but did not receive a distress call from M/T Wila," the official said.
One maritime expert suggested the ship was sailing under a Liberian flag.
Bloomberg said the vessel had been floating off the eastern coast of the UAE for the past month and appeared to have picked up a shipment in July near the Iraqi oil terminal of Basra.
The incident could rekindle some of the tension in the region from last year when ships were attacked in May and June near the Strait of Hormuz, the narrow channel between Iran and Oman through which about a quarter of the world's oil passes.
The US blamed Iran's Islamic Revolutionary Guard Corps for attacks believed to have been carried out with limpet mines and fast boats, damaging six commercial ships.
Earlier on Wednesday, the top US military commander in the region, Gen Frank McKenzie, described Iran as the “central problem” for the US in the Middle East.
The threat posed by Iran's proxies has forced US forces “to pull back”, said Gen McKenzie, the head of Centcom.
He said his country’s regional focus remained on countering Tehran.
“This headquarters focuses on Iran, executing deterrence activities against Iran, and doing those things,” he said at an event at the US Institute for Peace.