Iran's Islamic Revolutionary Guard Corps has seized a Panama-flagged oil tanker in the Strait of Hormuz, Tehran and Washington confirmed on Wednesday.
The Iranian navy confiscated the vessel but provided no further details, a report from the semi-official Tasnim news agency said.
"A dozen IRGCN [Iranian navy] fast-attack craft swarmed the vessel in the middle of the strait," said a statement from the Bahrain-based US Fifth Fleet.
"The IRGCN subsequently forced the oil tanker to reverse course and head toward Iranian territorial waters off the coast of Bandar Abbas, Iran."
Iran seized the vessel "following an order from the judiciary", the judiciary's Mizan Online news website said.
About 30 people were aboard the ship, including a Greek skipper and crew members from the Philippines and Sri Lanka, a spokeswoman for Greece's coastguard told AFP.
The Niovi oil tanker left Dubai and was travelling towards Fujairah when the fast-attack boats appeared, US Naval Command said.
The UK Royal Navy's Maritime Trade Operations said it had no comment.
It is the second time this week Iran has impounded an oil tanker near its waters.
On Thursday, the IRGC impounded a Marshall Islands-flagged tanker in the Gulf of Oman.
Both incidents have been condemned by the US, which says Iran is threatening regional security.
“Iran’s continued harassment of vessels and interference with navigational rights in regional waters are unwarranted, irresponsible and a present threat to maritime security and the global economy,” Washington said on Wednesday.
The Strait of Hormuz is often the site of tension between Iran and western powers.
Iran has seized several tankers near its waters amid heightened tension with the US.
The US Fifth Fleet patrols the area and recently sailed its first drone boat through the strait, a few months after it launched a special drone task force last year.
Iran briefly impounded several of the US drones being tested in the region in late August and early September, although there have been no similar incidents since.
The two seizures came after the US captured a Greek-managed tanker carrying Iranian oil, maritime security firm Ambrey said.
The Niovi's capture also followed a warning from Greek authorities of heightened risk, the firm added.
“Greek authorities had issued a warning that Greek shipping was at an increased risk from Iran after the detention of a Greek-managed Suezmax tanker carrying Iranian oil,” it said.
Iran has harassed, attacked or hampered the navigation of 15 foreign-flagged merchant vessels in the past two years, the US Fifth Fleet said, calling Tehran's actions “contrary to international law and disruptive to regional security and stability”.