US President Donald Trump has blamed Iran for attacks on two oil tankers at the entrance to the Gulf despite Tehran's denials, raising fears of a confrontation in the vital oil shipping route.
Iran has dismissed earlier US charges that it was behind Thursday's attacks that crippled two tankers. It has previously suggested it could block the Strait of Hormuz, the main route out for Middle Eastern oil, if its own exports were halted.
The blasts followed similar attacks a month earlier on four tankers, which Washington also blamed on Tehran.
"Iran did do it and you know they did it because you saw the boat," Mr Trump told Fox News.
He was referring to a video released on Thursday by the US military which said it showed Iran's Revolutionary Guards were behind the blasts that struck the Norwegian-owned Front Altair and the Japanese-owned Kokuka Courageous in the Gulf of Oman, at the mouth of the Gulf.
In a notable signal that close US allies are wary of Washington's position, Germany said the video was not enough to apportion blame for Thursday's attack.
"These accusations are alarming," Iranian Foreign Ministry spokesman Abbas Mousavi said.
Iran has accused the United States and its regional allies such as Saudi Arabia and the United Arab Emirates of "warmongering" by making accusations against it.
Asked how he planned to address Tehran and prevent any further incidents, Mr Trump told Fox News: "We're going to see." He also said any move to close the Strait of Hormuz would not last long.
He went on to call Iran a "nation of terror".
Nevertheless, Mr Trump, who last year pulled the United States out of an agreement between world powers and Tehran to curb Iran's nuclear programme in return for the lifting of sanctions, said that he was open to negotiations with Iran.
Iran has repeatedly said it will not re-enter talks with the United States unless it reverses the president's decision to withdraw from the nuclear deal.
Tehran and Washington have both said they have no interest in starting a war. But this has done little to assuage concerns that the two arch foes could stumble into a conflict.
China, the European Union and others have called for restraint from all sides.
UN Secretary-General Antonio Guterres called on Friday for an independent investigation of the attacks.
The tanker attacks took place while Prime Minister Shinzo Abe of Japan - a big buyer of Iranian oil until it was forced by the new US sanctions to stop - was visiting Tehran on a peacemaking mission, bringing a message from Mr Trump.
The US military said black-and-white footage it filmed from a US aircraft showed Iran's Guards on a patrol boat drawing up to the Kokuka Courageous and removing an unexploded limpet mine from its hull.
The Japanese-owned tanker, abandoned by its crew, was being towed to a port in the United Arab Emirates on Friday, after a Dutch firm said it had been appointed to salvage the ships.
The second tanker, the Front Altair, which was set ablaze by a blast, was still languishing at sea, although the fire that had charred the hull had been put out.
Iranian military fast-boats in the Gulf of Oman were preventing two privately owned tug boats from towing away the Front Altair, a US official claimed on Friday.