The United States has demanded Iran immediately release a tanker it seized in the Gulf, as a US military commander said the nation would work "aggressively" to ensure free passage of vessels.
Responding to an announcement by Iran's Revolutionary Guards that they had seized a foreign tanker smuggling fuel, the US State Department insisted Iran had to free the ship and its crew and stop harassing vessels in and around the Strait of Hormuz.
The US blames Iran for a series of attacks on shipping since mid-May in the world's most important oil artery, accusations Tehran rejects.
"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," a State Department spokesperson said in an email to Reuters on Thursday.
"Iran must cease this illicit activity and release the reportedly seized crew and vessel immediately."
It was unclear if the impounded ship was the same vessel that Iran towed to safety on Sunday after it sent a distress signal.
Iranian state television had earlier said it was the same ship but a Revolutionary Guards statement did not confirm that.
The Guards said the impounded ship was smuggling one million litres of fuel in the area of Larak Island in the Gulf and had 12 foreign crew.
Britain said the tanker was not British-flagged.
Reuters reported on Wednesday that shipping companies were hiring unarmed security guards for voyages through the Gulf as an extra safeguard.
Although Iran has yet to name the vessel, shipping industry sources said they believe it to be the MT Riah.
Refinitiv data showed that the last signal received from the vessel was on Sunday when it was in the Strait of Hormuz off the Iranian island of Qeshm, heading towards Oman from Larak Island.
US Central Command chief General Kenneth McKenzie said Washington was talking to several countries about ensuring freedom of navigation in the Gulf and would work "aggressively" to find a solution to enable free passage.
He was speaking in Riyadh at a news conference with General Prince Fahd bin Turki, commander of a Saudi-led coalition fighting the Iran-aligned Houthis in Yemen.
Washington has beefed up its military presence and the US Fifth Fleet, based in Bahrain, says Gulf Arab states have stepped up patrols.
Revolutionary Guards commander-in-chief Hossein Salami said Iran had adopted a defensive strategy but warned that "if our enemies make any mistakes .... our strategy can become an offensive one, " Iranian media reported.