WASHINGTON (Reuters) - The U.S. 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 U.S. defense official said on Thursday.

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

Reuters reported on Sunday that the United States 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 U.S. government as the most likely option for any formal response from Iran’s military.

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

The disclosure helps explain why the United States did not have Patriot air defenses deployed to locations like 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, U.S. forces took advantage of the hours of early warning provided by U.S. 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 U.S. casualties, an accomplishment that U.S. military leaders said was due to commanders on the ground - not Tehran’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 U.S. personnel at 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.

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

U.S. officials are still concerned that Iran-backed groups across the region could wage attacks on the United States.

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.