Al-Asad, Iraq (CNN) United States troops at the Al-Asad air base in Iraq were aware that an Iranian attack was imminent, allowing them to take shelter two-and-a-half-hours before missiles struck on Wednesday, CNN has been told during an exclusive tour of the devastated site.

Most troops were either flown out of the base or sheltering in bunkers by 11:00 p.m. local time Tuesday -- shortly before the first of four volleys of missiles began at just after 1:30 a.m. on Wednesday, officers said during the first tour by journalists of the air base.

A bunker at Al-Asad air base where US personnel sheltered.

The attack lasted around two hours, only targeting the US areas of the air base, which comprise around a quarter of the Iraqi base.

Officers called it a "miracle" that there were no casualties at the blasted site, with missiles landing just a few meters from bunkers, and some essential personnel remaining outside throughout.

It is the first time troops at the base have spoken in detail of the moments before the attack, revealing that they were able to seek shelter in bunkers shortly before missiles struck.

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