WASHINGTON - The fight for Ramadi is “pretty much over for now,” a US military official told an American television network on Monday, after Islamic State militants 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 IS control, the US official, who has seen the latest intelligence reports from Ramadi, told FOX News.

Although there were a large number of Iraqi Security Forces occupying Ramadi, most troops fled after IS fighters began their assault on the city centre Sunday, leaving behind Humvees and armoured vehicles supplied by the US military, another senior US military official told Fox News. “The Iraqi security forces were pushed out by a much smaller [IS] force,” the official said.

The takeover followed a three-day siege that began with a wave of IS car bombs and which dealt a devastating blow to the Baghdad government and the US forces providing logistical support. On Monday, Shia militias converged on the city, some 70 miles west of the capital, a bid to retake it. Earlier, the Pentagon stopped short of confirming the fall of Ramadi, saying the Islamic State militants had gained the advantage and that if the western Iraqi city fell, the US-led coalition would support Iraqi forces “to take it back later.” “Ramadi has been contested since last summer and ISIL now has the advantage,” Pentagon spokeswoman Elissa Smith said, using another acronym for Islamic State. She said the loss of the city would not mean the overall Iraq military campaign was turning in Islamic State’s favour, but acknowledged it would give the group a “propaganda boost.”

“That just means the coalition will have to support Iraqi forces to take it back later,” Smith said, adding that the United States continued providing it with air support and advice.

But Senator John McCain, a senior Republican leader, called the fall of Ramadi a “terribly significant” event that shows the need for more US forces on the ground.

“I think it’s, unfortunately, terribly significant, capital of Anbar Province, the deaths of hundreds, the displacement of thousands and thousands,” he said Monday on MSNBC’s “Morning Joe.”

“Not the 82nd Airborne, but we’ll have to have more people on the ground and this is really serious, the fall of Ramadi,” he said.

The defeat marks a blow to the Iraqi government and is a setback for the Obama administration, which has been training Iraqi forces to take back ground from the terrorist group.

McCain acknowledged a successful US special forces operation in Syria over the weekend that resulted in the killing of a top ISIS official and capture of his wife, as well as a trove of intelligence, but said raids were “almost peripheral if you’re going to lose the capital of the province.”“You have to give them the utmost praise. But to somehow assume that that will turn the tide here against ISIS, I think is just not realistic,” McCain said.

The US is training nine brigades of Iraqi government forces - including three Kurdish peshmerga brigades. However, Iraqi forces have needed the help of Shia militia fighters, the peshmerga, or US airpower to help beat back ISIS in different parts of the country.Part of the problem is that Anbar is predominantly Sunni, and Shia militia fighters - many of which are Iranian-backed and considered more effective - were not participating in Ramadi’s defence.

“Finally, the prime minister Abadi has said they are going to have to send Shia militia. You know, the Shia militia and Sunni and Anbar Province are absolutely enemies and Shia militia is now controlled by the Iranians,” McCain said.

McCain said fault lay with former Iraqi Prime Minster Nouri al-Maliki, for firing competent military leaders. But he also blamed President Barack Obama’s administration for withdrawing all US forces in Iraq in 2011.

“I hate to be repetitious, but the fact is that thanks to the surge, we had it under control and this is another consequence of the failure of this administration and this president to leave a residual force behind,” he said.

McCain said NATO ally Turkey has been reluctant to send forces or assist in the fight against ISIS since the US has so far been unwilling to target the Syrian regime under Bashar Assad, who it views as a greater threat than ISIS.

“Unless we are willing to allow these people -‚ for example, people trained to go in and take down (President) Bashar Assad, then they won’t cooperate,” he said.