The US is to send another 1,000 troops to the Middle East in response to what it describes as "hostile behaviour' by Iranian forces.
The move follows the release by the Pentagon of new images it says show Iran's Revolutionary Guard were behind the attacks on two oil tankers in the Gulf of Oman last week.
It said the photos, taken from a Navy helicopter, show Iranian forces removing an unexploded mine from the side of the Japanese-owned Kokuka Courageous.
Other photos show a large hole on the side of the Courageous, above the water line, that officials say appears to have been caused by another mine.
The Pentagon released the photos to bolster its claim that Iran was responsible for the attacks.
"Iran is responsible for the attack based on video evidence and the resources and proficiency needed to quickly remove the unexploded limpet mine," the US military's Central Command said in a statement.
In a statement announcing the additional deployment of troops, acting Defence Secretary Patrick Shanahan said: "The recent Iranian attacks validate the reliable, credible intelligence we have received on hostile behaviour by Iranian forces and their proxy groups that threaten United States personnel and interests across the region."
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He added that the extra troops were for what he described as "defensive purposes".
Iran insists the US has no hard evidence about its alleged involvement in the 13 June attacks on two ships in the Strait of Hormuz.
Foreign Secretary Jeremy Hunt has said the UK's own assessment was that Iran was behind the planting of limpet mines on the tankers.
Despite British and US intelligence findings, Germany and others insisted on Monday it needed a clearer picture before wading into a row which could have serious implications in the Middle East.