Iran must immediately release a vessel and crew it seized in the Arabian Gulf, the US State Department said on Thursday.
Iran’s Islamic Revolutionary Guard Corps admitted on state television that it took control of a foreign tanker near Larak Island in the Gulf, but claimed it had been “smuggling” fuel.
“The United States strongly condemns the Islamic Revolutionary Guard Corps Navy’s continued harassment of vessels and interference with safe passage in and around the Strait of Hormuz,” the State Department said.
“Iran must cease this illicit activity and release the reportedly seized crew and vessel immediately.”
The tanker and 12 of its crew were seized on Sunday in the strategic Strait of Hormuz, the Guard's Sepahnews website said.
It claimed that 1 million litres of fuel was aboard, and that it came from Iranian smugglers trying to sell to foreign customers.
Iran's English-language Press TV released video of the ship, showing its registration number that matched that of the Riah.
The UAE-based vessel turned off its location tracker as it entered Iranian territorial waters early on Sunday.
Iran earlier said it had been towed to Iranian waters for repairs.
The Guard said the impounded vessel, which Iranian authorities have not yet identified, was the same one it towed after it sent a distress call.
"The vessel that Iran towed to its waters after receiving a distress call, was later seized with the order from the court as we found out that it was smuggling fuel," the Guard said.
Britain said it was seeking further information about the seizure.
"We continue to urge the Iranian authorities to de-escalate the situation in the region," a government spokeswoman said.
"We are continuously monitoring the security situation there and are committed to maintaining freedom of navigation, in accordance with international law."
Tension is mounting between the US and Iran over the unravelling nuclear deal between Tehran and world powers.
On Thursday, Britain's Defence Minister Penny Mordaunt urged Iran to ease tension but vowed to continue to protect British interests in the Gulf.
US Central Command chief Gen Kenneth McKenzie said on Thursday that Washington was talking to other countries about freedom of navigation in the Gulf and would work "aggressively" to find a solution to enable free passage.
Gen McKenzie was speaking in Riyadh alongside Prince Fahd bin Turki, commander of the Saudi-led coalition fighting the Iran-aligned Houthis in Yemen.
Iran has threatened to respond to US sanctions on its economy after Washington withdrew from the 2015 nuclear deal, in which Iran agreed to scale back its uranium enrichment in return for the lifting of sanctions.
It also warned of consequences after the British seized a tanker believed to be carrying Iranian oil to Syria, in breach of EU sanctions.
During a call with French President Emmanuel Macron on Thursday, Iranian President Hassan Rouhani urged Europe to speed up efforts to salvage the 2015 deal.
"Europe must accelerate its efforts to realise Iran's legitimate interests and bring about a ceasefire in the US economic war" against Iran, Mr Rouhani told Mr Macron.
"Iran is determined to keep open all options" to save the landmark deal, he said.
He also warned Mr Macron that "missed opportunities" would force Iran to drop further commitments under the deal, after it withdrew from some commitments on its nuclear programme in May.