Investigators are not sure exactly what tore the ten-foot hole in the hull of the Andrea Victory, a Norwegian oil tanker, as it anchored off the coast of the United Arab Emirates.

One theory is that scuba divers swam through the dark waters of the Gulf of Oman and planted explosive charges on the ship, as well as a Saudi tanker and two other vessels.

Another possibility is that underwater drones darted between the giant oil tankers and struck them from below. Either way, Western officials are increasingly convinced they know who was behind it.

“The Iranians are lucky this didn’t go better,” one said. “If a Saudi oil tanker went down we could be looking at war.”

Severe enough to cause alarm, but small enough to avoid sparking a war, the sabotage mission near the mouth of the Persian Gulf typifies the strange flare up between Iran and the United States over the past fortnight.

The United States says it has intelligence that Iran is plotting strikes against its assets in the middle East. But officials say it is unlikely the Islamic Republic would carry out a blatant assault against a superpower with superior weaponry.

The Andrea Victory was one of four ships sabotaged off the UAE credit: ALI HAIDER/EPA-EFE/REX

Instead, Iran’s Revolutionary Guard Corps is believed to be looking for ways to strike at the US and its Arab allies while concealing its own involvement.

Another tool are Iranian-backed Shia militia groups in Iraq, who are able to operate near American forces and diplomatic facilities in Iraq and could be ordered to carry out mortar attacks, shootings, or even kidnaps against US personnel.

In recent weeks Iran have activated two groups - Asa’ib Ahl al-Haq and Kata'ib Hizbollah - to potentially carry out attacks in Baghdad, according to Phillip Smyth, an analyst at the Washington Institute.

“The Iranians may be looking to send some very, very strong signals America’s way and prove themselves. The other option is they are being more aggressive because they want to test things and understand what the true American red line is,” Mr Smyth said.

A map tracking Shia militas (Phillip Smyth / Washington Institute)

Over the past two weeks the US has deployed an aircraft carrier and a heavy bomber squadron, advised its citizens to avoid all travel to the region, and withdrawn diplomats from Iraq.

And in a chilling reminder of how dangerous a confrontation could be, US diplomats on Saturday warned that civilian aircraft flying near the Gulf should be wary of the risk of being "misidentified."

Their statement recalls a similar confrontation in 1988, which ended in tragedy when a US warship shot down an Iranian airliner that it mistook for an attack jet, killing 290 people.

An Iraqi oil official on Saturday said employees of energy giant Exxon Mobil had started evacuating an oil field in the southern province of Basra.

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Bahrain also ordered citizens in Iraq and Iran to leave the countries on Saturday, citing instability in the region.

In recent days both sides appear to have stepped back from the brink, with top officials easing their rhetoric.

Javad Zarif, the Iranian foreign minister said on Saturday that "there will be no war because neither we want a war, nor has anyone the idea or illusion that it can confront Iran in the region."

And standing at the portico of the White House on Thursday, Donald Trump was asked by reporters if there was going to be a war. "Hope not," he replied.

John Bolton has taken a hawkish line credit: JIM LO SCALZO/EPA-EFE/REX

In private meetings, Mr Trump has reportedly been more forceful, telling his defence officials that he does not want to see the US dragged into another confrontation in the Middle East and voicing frustration at the escalating tensions.

At the same time, Mr Trump has not reined in his hawkish national security advisor, John Bolton, who has repeatedly called for regime change in Tehran and put his own name on the statement announcing a US aircraft carrier was being sent to the Gulf.

“Although President Trump certainly doesn't want a new war of choice in the Middle East, he has surrounded himself with a war cabinet who has been itching for a fight with Iran for years,” said Ali Vaez, director of the Iran Project at the International Crisis Group.

In May last year, Mr Trump pulled the United States out of the 2015 agreement that offered Iran economic relief in exchange for curtailing its nuclear programs.

An Iraqi official said Exxon had begun evacuating employees from the Nihran Bin Omar oilfield near Basra credit: Nabil al-Jurani/AP

Since then, the team led by Mr Bolton has pursued a policy of "maximum pressure", tightening economic sanctions in a bid to force Tehran into a more restrictive deal also addressing its missile program and regional meddling.

Iran this month struck back by saying it would ditch its own commitments under the nuclear deal in July if the world did not act to ease the impact of the US sanctions.

No government has yet officially accused Iran of being behind the attack at Fujairah, the anchorage near the strategic Strait of Hormuz.

Tehran is treating the incident with a mixture of denials and jibes that longtime observers recognise as a trademark of Iranian policy.

While Iran’s foreign ministry condemned the attack as “dreadful” and called for an investigation, the Kayhan newspaper, which is close to Iran’s supreme leader, said the explosions were “a wake up call for the reactionary sheikhs” in Saudi Arabia and the UAE.

For now, it seems that almost no one - including Mr Trump, Iran’s leadership, Saudi Arabia, Israel, or the EU - is eager for a fullblown conflict. But that does not mean there will not be one, either through miscalculation or misunderstanding.

“What we need is a period of calm, so that everyone understands what the other side is thinking,” said Jeremy Hunt, the foreign secretary, said this week. “This is already the most unstable region in the world."