The US military has released video footage which it says shows an Iranian military patrol boat approaching one of two tankers attacked in the Gulf of Oman, to support the Trump administration’s claims that Iran was responsible.

The blurry black and white footage, taken from the air, shows a small military boat alongside the Japanese-owned Kokuka Courageous, and someone standing up on the prow of the boat to remove an object from the tanker’s hull. The boat then pulls away from the tanker.

US officials were quoted as saying the boat was an Iranian Revolutionary Guard patrol boat approaching the tanker after it was attacked on Thursday, and the object removed was an unexploded limpet mine. It was unclear whether it was being alleged the Iranian sailors were detaching the mine in order to remove evidence.

Thursday’s attacks increased tension between the US and Iran, which are locked in a standoff after Washington imposed economic sanctions on Tehran in an attempt to force the renegotiation of the 2015 Iran nuclear deal.

The incidents also added to security fears over oil tankers travelling through the strait of Hormuz – a chokepoint for the global oil and gas trade – and appeared to have ended a Japanese-led mediation effort between Tehran and Washington.

The US military also released a photo it claimed showed a mine on the side of the Kokuka Courageous and some damage to the hull.

Facebook Twitter Pinterest Photograph: US Central Command

The operator of the tanker suggested the vessel was struck by a missile. The crew saw “flying objects” just before it came under attack, according to Yutaka Katada, the president of the Tokyo-based owner, Kokuka Sangyo.

The vessel’s crew had spotted an Iranian naval ship nearby, he added, but did not say whether that was before or after the attack, which damaged the tanker’s starboard side.

Donald Trump called Iran a "nation of terror” on Friday and said the US had “exposed” Iran’s responsibility for the attacks. “Iran did do it,” the US president said in an interview with Fox & Friends.

On Thursday Mike Pompeo said the US believed the blasts were caused by the Iranian military “based on intelligence, the weapons used, the level of expertise needed to execute the operation”. The US secretary of state claimed the attacks had such a high degree of sophistication they could not have been carried out by a proxy.

Pompeo took no questions after making his allegations. He said Iran had been responsible for previous attacks on tankers last month, but the official investigation into those incidents did not determine who had been responsible.

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Tehran denied all responsibility for Thursday’s attacks and its foreign minister, Javad Zarif, suggested others could be trying to provoke a conflict between Iran and the US.

On Friday he tweeted that the US had “immediately jumped to make allegations against Iran without a shred of factual or circumstantial evidence”.

The incident took place on a day when Iran’s supreme leader, Ayatollah Ali Khamenei, bluntly rejected the proposal of a resumption of US-Iranian talks, as suggested by Japan’s prime minister, Shinzo Abe, on a visit to Tehran. Abe is widely thought to have had the blessing of Trump in offering to open a channel of communication between the US and Iranian leaders.

But after Khamenei’s rejection and the tanker attacks, the US president followed suit, posting a tweet saying: “I personally feel that it is too soon to even think about making a deal. They are not ready, and neither are we!”

The blasts on the Kokuka Courageous and the Norwegian-owned Front Altair pushed oil prices up by 4% and could further raise insurance premiums for vessels operating in the Gulf.

Crew members from both vessels were rescued. One Kokuka Courageous crew member sustained burns on his hands and was treated by a US navy medical team.

Iran’s English-language Press TV aired footage of rescued crewmen from the Front Altair, saying they are all in “full health”.

“Everything is OK,” said one of the vessel’s “chief officers”, presenting himself as Russian and thanking Iran for its “hospitality”.

Press TV said 11 of the crew were Russian, 11 Filipino and one Georgian.

Facebook Twitter Pinterest Oil prices rose 4% after the attack. Photograph: Reuters

Confirming the involvement of its ships, US Central Command issued a statement saying it would not tolerate any attempt to interfere in its operations but insisted it was not interested in conflict.

Saudi Arabia supported Pompeo’s claims. Its foreign minister, Adel al-Jubeir, said on Thursday: “We have no reason to disagree with the secretary of state. We agree with him.”

The UK foreign secretary, Jeremy Hunt, said his assumption was that the intelligence assessment of the US, Britain’s closest allies, was correct, but that the UK would make its own inquiries. Hunt said he was “very worried that both sides in the crisis think the other side does not want war. The risk you have is that they do something provocative that leads to unintended catastrophic consequences.”

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Khamenei dismissed the notion of talks with the US, saying in a series of tweets: “I do not consider Trump as a person worth exchanging any message with and I have no answer for him, nor will I respond to him in the future.”

He said Iran had no plan to build nuclear weapons, but if it wished to do so, the US would be unable to stop it.

Pompeo said the attacks on the tankers were intended as an insult to the Japanese leader. “Iran’s supreme leader rejected Abe’s diplomacy today by saying he has no response to President Trump and will not answer,” he said.

The US put its allegations before a closed session of the United Nations security council on Thursday. Iran responded by accusing the US of stoking tensions and mounting an “economic war” against its people.