The US military has released a video which it says shows Iran's Revolutionary Guard removing an unexploded mine from the side of an oil tanker damaged in an attack in the Gulf of Oman on Thursday.
US officials also shared a photo of the Japanese tanker, apparently showing the unexploded mine before it was removed.
A Norwegian tanker was also damaged.
The US accused Iran of being behind the mine attacks. Iran said it "categorically rejects" the allegation.
- Gulf of Oman tanker 'attacks': What we know
- Iran, the US and the Gulf: What now?
- US-Iran tensions explained
The US said the footage showed Iran's Revolutionary Guard "removing [an] unexploded limpet mine" from the side of the Kokuka Courageous ship more than eight hours after the blasts occurred.
In a statement released on Friday, the Iranian mission to the United Nations said that it condemned the allegation "in the strongest possible terms".
The blasts came a month after four oil tankers were damaged in an attack off the coast of the United Arab Emirates. The US blamed Iran for that attack, but did not produce evidence. Iran denied the accusations.
Oil prices jumped as much as 4% after Thursday's incident in the Gulf of Oman, which lies at one end of a vital shipping lane through which hundreds of millions of dollars' worth of oil pass every year.
What did Mike Pompeo say?
"It is the assessment of the United States that the Islamic Republic of Iran is responsible for the attacks," the US secretary of state said at a news conference in Washington.
"This assessment is based on intelligence, the weapons used, the level of expertise needed to execute the operation, recent similar Iranian attacks on shipping, and the fact that no proxy group operating in the area has the resources and proficiency to act with such a high degree of sophistication."
"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."
What do we know about the explosions?
The Norwegian Maritime Authority said on Thursday that there had been three blasts on board the Front Altair.
Wu I-fang, a spokesman for Taiwan's CPC Corp oil refiner, which chartered the Front Altair, said it was carrying 75,000 tonnes of naphtha.
The operator of the Kokuka Courageous, BSM Ship Management, said its crew abandoned ship and were rescued by a passing vessel.
Both Iran and the US have released pictures showing rescued crew members on board their vessels.
Why are US-Iran tensions so high?
In 2018, the US pulled out of the landmark nuclear deal reached in 2015 that was aimed at curbing Iran's nuclear activities.
The move was strongly criticised by a number of countries, including America's closest allies.
In May, President Donald Trump tightened US sanctions on Iran - mainly targeting its oil sector.
Iran then announced it was suspending some commitments under the nuclear deal.
In recent months the US has strengthened its forces in the Gulf - saying there was a danger of Iranian attacks.
It has sent an aircraft carrier strike group and B-52 bombers to the region. In response, Iran has accused the US of aggressive behaviour.
Those tensions rose markedly after the 12 May limpet mine attacks in the UAE.
The UAE blamed an unnamed "state actor". The US said that actor was Iran, an accusation Tehran has denied.
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