The United States knew Iranian missiles were going to strike American facilities in Iraq well before the actual attacks, according to a report published by Washington Post, quoting several defence and White House officials.

Officials say the early warning came from intelligence sources as well as from communications in Iraq.

“We knew, and the Iraqis told us, that this was coming many hours in advance,” said a senior administration official, who like others quoted in the report spoke on the condition of anonymity. “We had intelligence reports several hours in advance that the Iranians were seeking to strike the bases," the senior administration official said.

Read more: US-Iran conflict: Before and after the killing of Gen Soleimani

On the other hand, a senior defence official downplayed Iraq's role in the matter, stating that if they [Iraqis] had provided a warning, it was not hours in advance.

According to the publication, US Defence Secretary Mark Esper had convened a meeting with Chairman of the Joint Chiefs of Staff Gen Mark Milley along with senior civilian leaders of the department before being called out after receiving a notification that the strikes were under way.

The Post says that the advance warning gave military commanders enough time to get US troops into bunkers and dressed in protective gear. It added that the troops remained in their protected hideouts for hours after the strike.

One official stated that at least some troops left the al-Asad air base in Western Iraq before the attack.

“It’s not luck that no one got killed,” a second senior defense official said. “Luck always plays a role. But military commanders on the ground made good judgment and had good response.”

In a televised address after the attack, US President Donald trump acknowledged the role played by "an early warning system” and credited it for preventing loss of life. According to a defence official, the president was referring to a radar network the US military has to search for enemy missiles.

Read more: US-Iran row: In call with US Defence Sec, COAS stresses need for 'diplomatic engagement'

In light of this information, the report maintained that at least two intelligence sources gave the US time to hatch up a plan.

First, there were indications that Iran was preparing to strike US troops in Iraq before the launch, although it was not clear where this information came from.

Another official said that the US military had “clear indications” from information “internal to the government”. "The Pentagon fully expected a retaliation from Iran,” the senior defence official was quoted as saying. “What that was was the issue, but we fully expected some sort of reaction.”

The second warning came from "technical means". According to the report, the US military has satellites that can detect a missile shortly after it is launched, which alerted the US government about the strike.

As a precautionary measure after ordering the strike, which killed Soleimani, US military deployed around 4,500 soldiers to the Middle East and also shuffled forces within the region. In addition, commanders on the ground also moved service members off small bases, scattering equipment and people to make them harder to target.

Read more: 'Hoping for de-escalation': Alarm and concern following Iran missile attacks on US bases in Iraq

“Let’s get people out of less defensible areas and put them in more easily defended or better- defended areas,” said one of the senior defense officials. “But at the same time, let’s not overly mass our personnel as a single target.”

Side by side, US officials began alerting the media about a possible Iranian strike beginning at 4pm on Tuesday, approximately an hour before they actually took place.

According to the report, US military officials were not sure which bases Iran had targeted once the missile strike was launched.

“The attack spread out for more than an hour [...] It was more than an hour from the first attack to the last attack. This was not a ‘boom’ and all of this hit at once. This was launch, launch, launch," said the senior defence official.

The report goes on to say that once the missile strike was launched, there was constant communication among the White House, Central Command and two other combatant commands: Northern Command and Strategic Command.

Once the missiles made contact, US military officials began to assess the damage.

The Pentagon were quick to call their allies and several partner nations to communicate in the wake of the strike that killed Soleimani. While some raised questions about the US' strategy post-Soleimani, they were supportive and "grateful" for the information, the Post reported.

Therefore, by 7:30pm on Tuesday, White House officials had briefed Trump and were able to say confidently that there were no American casualties.

Read more: Commercial airlines reroute flights amid US-Iran tensions

However, officials did not know for certain about the fatalities until Wednesday when service members were able to assess the wreckage.

The subsequent lack of casualties boosted officials' belief that the Iranians had intended to make a "public show of force to save face", said the defence official.

Esper and US State Secretary Mike Pompeo arrived at the White House at 7pm on Tuesday. An hour later, Trump began ringing up lawmakers to inform them that no American had been killed in the missile strikes, the senior administration official was quoted as saying.

“Our initial reaction has been, this was a domestic effort from the Iranians to save face, not to go to war, so we have proceeded in that vein,” said another senior administration official with knowledge of the analysis.