Fears that Iran was about to attack American targets in the Middle East were sparked by pictures of fully-assembled missiles loaded on to the back of boats in the Persian Gulf, it has been claimed.
Iranian paramilitary forces were seen loading the weapons on to small craft, amid concern that the Revolutionary Guards would fire them at US navy vessels.
Additional intelligence reported threats to commercial ships and potential attacks by Iranian-backed militias on American troops in Iraq, which, taken together, led the US to believe an attack was imminent.
American warnings of a 'credible threat' against its forces in the Middle East have sent tensions across the region soaring, as Arab nations including Saudi, UAE and Iran insisted they were ready for a war but didn't want one.
Overhead images of fully-assembled Iranian missiles on boats in the Persian Gulf sparked fears they would be fired at US ships and prompted warnings of a 'credible threat' (pictured, USS Abraham Lincoln carrier group on May 8, on its way to Arabian Peninsula)
US embassy personnel were evacuated from Iraq as the Abraham Lincoln carrier group (pictured) was moved to the region, sending tensions soaring
The photographs in particular presented a new kind of threat to the one previously seen from Iran, prompting embassy evacuations on Wednesday, three security sources told the New York Times.
President Trump's security team, and in particular National Security Adviser John Bolton, took the threat seriously enough to order the withdrawal of non-essential embassy staff from Iraq, which has been completed.
Tensions between the US and Iran, which have been building for months, peaked in recent days following attacks on oil tankers and pumping stations that Washington has blamed on Tehran.
American security experts believe Iran gave its 'blessing' to tanker attacks, which hit two Saudi crude oil tankers, a UAE-flagged fuel bunker barge and a Norwegian-registered oil products tanker off Fujeirah near the Strait of Hormuz.
The source said the United States believes Iran's role was one of actively encouraging militants but indicated the United States does not now have evidence that Iranian personnel played any direct operational role.
The head of Iran's Revolutionary Guards (pictured) insisted he was 'fully prepared for a confrontation with the enemy', despite Ayatollah Khameni saying Iran did not want conflict
Houthi rebels, who are back by Iran, have already claimed responsibility for the attacks on two Saudi oil pumping stations earlier this week.
Iran's Foreign Ministry has called the tanker attacks 'worrisome and dreadful' and called for an investigation.
The precariousness of the situation was underlined Wednesday as multiple nations emphasised that they are ready for war, while insisting it is the last thing they want.
Iran's Revolutionary Guards said it is 'fully prepared for a confrontation with the enemy' and was backed by defence minister Amir Hatami who insisted 'we will defeat the United States' in any military confrontation.
Meanwhile Saudi Arabia struck