The fight for Ramadi is “pretty much over for now,” a U.S. military official told Fox News, after ISIS overran the beleaguered Iraqi army to take control of the key city Sunday, killing an estimated 500 soldiers and civilians.

Iraqi security forces abandoned their Anbar Operations Center in Ramadi overnight, leaving the city almost completely in ISIS control, according to the U.S. official, who has seen the latest intelligence reports from Ramadi.

Although there were a large number of Iraqi Security Forces occupying Ramadi, most troops fled after ISIS fighters began their assault on the city center Sunday, leaving behind Humvees and armored vehicles supplied by the U.S. military, a separate senior U.S. military official told Fox News.

"The Iraqi security forces were pushed out by a much smaller [ISIS] force," the official said.

The takeover followed a three-day siege that began with a wave of ISIS car bombs and which dealt a devastating blow to the Baghdad government and the U.S. forces providing logistical support. On Monday, Shia militias converged on the city, some 70 miles west of the capital, a bid to retake it.

The retreat by Iraqi forces was reminiscent of the nation's earliest battles against ISIS, including the fall of Mosul, when poorly trained Iraqi soldiers shrank from the black-clad Islamist army, leaving guns and other gear behind for the terrorists to capture. In Ramadi Sunday, bodies littered the streets as local officials reported the militants carried out mass killings of Iraqi security forces and civilians. Online video showed Humvees, trucks and other equipment speeding out of Ramadi, with soldiers gripping onto their sides.