The Isil extremist group seized control of the key city of Ramadi yesterday, sending Iraqi forces racing out of the city and bringing Isil close to the capital Baghdad.
Online video showed Humvees, trucks and other equipment purportedly speeding out of Ramadi, with some soldiers gripping onto their sides.
Prime Minister Haider al-Abadi ordered security forces not to abandon their posts across Anbar province, apparently fearing the extremists could capture the entirety of the vast Sunni province that saw intense fighting after the 2003 US-led invasion of the country to topple dictator Saddam Hussein.
The retreat recalled the collapse of Iraqi police and military forces last summer, when the Isil group's initial blitz into Iraq saw it capture about a third of the country.
It also calls into question American officials' hopes of relying solely on airstrikes to support the Iraqi forces in expelling the extremists.
"Ramadi has fallen," said Muhannad Haimour, a spokesman for the governor of Anbar province. "The city was completely taken... It was a gradual deterioration. The military is fleeing."
The final push by the extremists began early yesterday, when police and army officials said four nearly simultaneous bombings targeted police officers defending the Malaab district in southern Ramadi, killing 10 and wounding 15. Among the dead was Col Muthana al-Jabri, the chief of the Malaab police station.
Later, three suicide bombers drove their explosive-laden cars into the gate of the Anbar Operation Command, the military headquarters for the province, killing five soldiers and wounding 12.
Fierce clashes erupted between security forces and Isil militants after the attacks.
Isil militants later seized Malaab after government forces withdrew, with the militants saying they now held the military headquarters.
A police officer who was in Malaab said retreating forces left behind about 30 army vehicles and weapons that included artillery and assault rifles. He said some two dozen police officers also went missing during the fighting.
On a militant website frequented by Isil members, a message from the group claimed its fighters held the 8th Brigade army base, as well as tanks and missile launchers left behind by fleeing soldiers.
The new setback came only a day after Baghdad's decision to send reinforcements to help its battered forces in Ramadi.
The military had also issued a statement calling on its forces not to abandon Anbar province.
"Victory will be on the side of Iraq because Iraq is defending its freedom and dignity," the military said.
Last week, the militants swept through Ramadi, seizing the main government headquarters and other key parts of the city.
It marked a major setback for the Iraqi government's efforts to drive the militants out of areas they seized last year.
Previous estimates suggested the Isil militant group held at least 65pc of the vast Anbar province.