The US military released a video on Friday that it says shows Iran's Revolutionary Guard removing an unexploded mine from one of the oil tankers targeted near the Strait of Hormuz.
It suggested the Iran had sought to remove evidence of its involvement from the scene.
US Central Command also released photographs showing the apparent mine, which attaches to the side of a ship magnetically.
Release of the classified black-and-white images showed US determination to convince the international community that Iran was responsible for Thursday's attacks on the Japanese-owned Kokuka Courageous, and the Norwegian-owned Front Altair.
However, the Japanese operator of Kokuka Courageous said the crew saw "flying objects" just before the attack, suggesting the tanker was damaged by something other than mines. Yutaka Katada, the company president, said reports of a mine attack were "false".
Both vessels suffered explosions, forcing crews to abandon ship and leave them adrift in waters between Gulf Arab states and Iran. All crew members were safely evacuated.
Navy Captain Bill Urban, a Central Command spokesman, said the Revolutionary Guard vessel was observed at 4.10pm local time approaching the Kokuka Courageous.
He said: "It was observed and recorded removing the unexploded limpet mine from the Kokuka Courageous."
He added: "The United States has no interest in engaging in a new conflict in the Middle East. However, we will defend our interests."
Mike Pompeo, the US secretary of state, said: "Taken as a whole these unprovoked attacks present a clear threat to international peace and security, a blatant assault on the freedom of navigation and an unacceptable campaign of escalating tension by Iran".
Mr Pompeo said American assessment of Iran's responsibility was based on intelligence sources, the weapons used and the level of expertise, saying "no proxy group operating in the area has the resources and proficiency to act with such a high degree of sophistication".
In a statement the Iranian mission to the United Nations said Tehran "categorically rejects the US unfounded claim with regard to 13 June oil tanker incidents and condemns it in the strongest possible terms."
It accused the United States and its allies, including Saudi Arabia, of "warmongering."
Jeremy Hunt, the foreign secretary, said the UK believed the US assessment that Iran was responsible but repeated his warning there was a serious danger of the US and Iran stumbling into an unwanted war.
"We have no reason not to believe the American assessment and our instinct is to believe it because they are our closest ally," Mr Hunt told the Today programme.
"We are very worried about the situation in Iran because at the moment both sides in this dispute think the other side doesn't want war and the risk you have is that then they doing something provocative that leads to catastrophic consequences that weren't intended."
The explosions, which left one of the oil tankers burning outside the strategic Strait of Hormuz waterway, marked the most serious incident since the White House warned in early May that Iran was plotting attacks in the region.
The incident came one month after Iranian forces allegedly used naval mines to blow holes in two oil tankers and two smaller ships off the Emirati port of Fujairah. The US publicly said Iran was behind that attack, while Tehran denied responsibility.
Javad Zarif, the Iranian foreign minister, said "suspicious doesn't begin to describe" the incident in Gulf of Oman. He previously suggested without evidence that Israel was staging the attacks to undermine Iran.
Responding to Mr Zarif's comments, Mr Pompeo said: " Foreign Minister Zarif may think this is funny but no one else in the world does."
The explosions at sea came hours before Shinzo Abe, Japan's prime minister, met with Ayatollah Khamenei, the supreme leader of Iran, on a diplomatic mission intended to try to ease tensions between Iran and the US.
But the Japanese leader's efforts appeared to bear little fruit. Ayatollah Khamenei refused to hear any messages from Donald Trump delivered by Mr Abe, the Iranian government said. The ayatollah also said Iran was not seeking nuclear weapons but "America could not do anything" to stop Tehran if it did decide to pursue a nuclear course.
Mr Trump on Thursday ruled out the possibility of making a deal with Iran, saying: "While I very much appreciate PM Abe going to Iran to meet with Ayatollah Ali Khamenei, I personally feel that it is too soon to even think about making a deal. They are not ready, and neither are we!"
Both tankers were carrying "Japanese-related" cargo, according to Japan's government. It was not clear if that was a coincidence or if the targeting of the ships was done deliberately to coincide with Mr Abe's visit to Tehran.
While Iran's civilian government denies responsibility for any of the attacks, it is possible that the Revolutionary Guard, who answer directly to the supreme leader, are carrying out operations without the government's knowledge or consent.
Iran tries to keep tensions between the two sides concealed but they spilled into the open early this year when Mr Zarif threatened to resign after he was left out of a key meeting while a senior Revolutionary Guard commander was invited.
The attacks capped six weeks of building tensions between Iran and the US during which time Mr Trump has ordered an aircraft carrier, a bomber taskforce and 1,500 additional troops to the Middle East. Both the US and Iran say they are not looking for war but Britain and other countries have warned of the danger that the two sides could stumble into an unintended conflict.
Antonio Gutteres, the UN secretary general, condemned the attack and warned that the world cannot afford "a major confrontation in the Gulf region".
Any fighting near the Strait of Hormuz, the waterway which transports 20 per cent of the world's oil, would likely cause serious damage to global energy supplies.
Analysts said that Iran appeared to be lashing out in order to send a message in response to crippling US sanctions imposed by Mr Trump after he withdrew the US from the 2015 nuclear deal.
"I think Iran is showing that it has teeth," said Charles Hollis, a former British diplomat in Tehran who is now managing director of the Falanx Assynt consultancy. "It's a way of showing that if they are backed into a corner they are not without means of causing grief."
Iran has warned that it will begin enriching high-grade uranium, the kind that could be used for a nuclear weapon, in July unless Europe finds a way to get around US sanctions and prop up the Iranian oil and banking sectors.