WASHINGTON (REUTERS) - The United States military is weighing adjustments to its defensive posture in the Middle East after Iran upended assumptions by staging a missile attack in Iraq, a country where it wields influence, a US defence official said on Thursday (Jan 9).

For days before Wednesday's unprecedented strike, Iran had threatened to retaliate over the US killing of Iranian military commander Qassem Soleimani - threats that Washington took seriously.

Reuters reported on Sunday that the US had detected that Iranian missile forces across the country had been put on a heightened state of alert. A missile attack had been seen inside of the US government as the most likely option for any formal response from Iran's military.

But Teheran had been seen as more likely to attack US positions in countries other than Iraq, where Teheran counts some influential allies, the senior US official said, speaking on condition of anonymity.

The disclosure helps explain why the US did not have Patriot air defences deployed to locations like Ain Al-Asad air base in Iraq, where at least 11 of Iran's ballistic missiles struck in Wednesday's attack. Such systems are deployed elsewhere in the region where American forces are stationed, including in Saudi Arabia, an arch-foe of Iran.

Instead, US forces took advantage of the hours of early warning provided by US intelligence and were able to take more rudimentary defensive measures before missiles fired from at least three locations inside of Iran hit their targets in Iraq.

Such precautions include "scatter plans", huddling in bunkers and protective gear to help shield American forces that come under fire.

There were no US casualties, an accomplishment that US military leaders said was due to commanders on the ground - not Teheran's goodwill.

Army General Mark Milley, the chairman of the Joint Chiefs of Staff, has said he believed the attack had been intended to kill US personnel at Ain Al-Asad. He noted that the missiles had 1,000- to 2,000-pound warheads on them, each with significant explosive power able to kill people in a wide area around the detonation site.

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It was unclear whether the US military might now seek to station Patriots inside of Iraq - and where they would be moved from. Air defences are a scarce resource in the US military.

US officials are still concerned that Iran-backed groups across the region could wage attacks on the US.

The Pentagon has sent thousands of additional forces to the Middle East in recent weeks, including from the 82nd Airborne Division and the 173rd Airborne Brigade.