The sophistication of the attacks on two shipping tankers in the Gulf of Oman last week had already led most independent analysts to conclude Iran was responsible for the high-profile explosions.

But there has been scepticism from some key countries, including Germany and Japan, after the US initially released a grainy black and white video it said showed Iranian forces removing an unexploded mine from one of the two targeted ships. Iran has denied involvement.

The fresh set of colour images released by the US Department of Defense overnight add a little more visual clarity to the diplomatic debate, including an image of an armed speedboat with troops in red life-jackets on board, and life-jacketed personnel by the side of the Japanese vessel that was struck last week.

A photo the US says shows an Iranian patrol boat after reportedly removing an ‘unexploded limpet mine’. Photograph: AFP/Getty Images

But the principal evidence used to justify Iranian involvement has been the method employed. The two tankers were mined above the waterline – damaging them, but not sinking them – prompting the conclusion the actions were designed to make a point. There were no fatalities in either case, with both crews rescued.

Jack Watling, a research fellow at the Royal United Services Institute thinktank, said the attacks were “very precise, to avoid the risk of an oil spill”, to make the point that “Iran can impose a cost on the United States, and that the US can’t police the waterway around the strait of Hormuz”.

Q&A Why is the Gulf of Oman so important for shipping oil? Show The strait of Hormuz, which provides passage from the Gulf of Oman to the open sea, is the most important gateway for oil exports in the world. With Iran on its northern shore, and the UAE and Oman on its southern shore, the US Energy Information Administration (EIA) calls it the world’s worst 'chokepoint' In 2016, 18.5m barrels of crude oil were transported each day through the strait of Hormuz, compared with 16m through the strait of Malacca, which runs between the Indonesian island of Sumatra, Malaysia and Thailand, connecting the Indian Ocean with the South China Sea. 5m barrels of crude oil are transported annually through the next largest chokepoint, the Suez canal. Phillip Inman

Both ships hit last Thursday were operating 19 and 21 nautical miles off Iran – just beyond the 12-mile territorial limit – in the crowded waterways through which 40% of the world’s internationally traded crude passes. Oil prices rose 4.5% on the day.

The mines used were small. Another photo released by the US shows a limpet mine attached to the Japanese vessel, the Kokuka Courageous, which did explode, creating a hole 1.5 metres high and 1.1 metres wide; dramatic enough when pictured, but again not enough to threaten the ship.

A hole blasted in the side of the Kokuka Courageous. Photograph: US Department Of Defense/EPA

The wider argument used to justify Iran’s involvement is that it would make sense as a response by the country to what it perceives as the economic warfare waged by the US in withdrawing from the nuclear deal and imposing sanctions. “The status quo for Iran is unacceptable,” Watling said.

David Wearing, a teaching fellow at Royal Holloway and Bedford New College, cautions that the US and its regional allies are “not reliable narrators”, partly because it is in Washington’s interest “to narrow debate to the question of whether or not Iran is guilty of these attacks to present their own aggression as defensive”.

But he acknowledged the evidence so far suggested it was probable Iran was directly involved. “They would likely be attempting to strengthen their hand in this standoff by demonstrating their ability to impose costs on their opponents,” the analyst added.

An image the US has said shows an unexploded limpet mine used in an attack on the Kokuka Courageous. Photograph: US Department Of Defense/EPA

The release of the images was followed by the announcement that the US was sending another 1,000 troops to the Middle East, a modest show of strength designed to reinforce Washington’s argument about culpability.

A British assessment has also concluded Iranian forces were responsible for the tanker attacks, but there is not understood to be any immediate plans for the UK to step up its military commitment to the region.

Watling believes Tehran can avoid further recrimination “as long as there is sufficient ambiguity”, partly because few believe that Donald Trump, who campaigned on an isolationist agenda, wants war – and because the backing obtained from other countries remains limited.