The Iraqi city of Ramadi has fallen to Islamic State (IS) after government forces abandoned their positions, officials say.
The police and military made a chaotic retreat after days of intense fighting.
But the US refused to confirm the capture, saying the situation was "fluid and contested" and it was too early to make "definitive statements".
Ramadi is the capital of Iraq's largest province, Anbar, and is just 70 miles (112km) west of Baghdad.
A statement purportedly from IS said its fighters had "purged the entire city". It said IS had taken the 8th Brigade army base, along with tanks and missile launchers left behind by troops.
A very well-placed source in the Anbar governor's office told the BBC Ramadi was now under the full control of Islamic State, and all government troops had withdrawn.
Prime Minister Haider al-Abadi had ordered troops to stand their ground, saying he was deploying Shia militia to the city.
But an army officer told the BBC that most troops had retreated to a military base in the city of Khalidiya, east of Ramadi.
Government troops were running out of ammunition and could not repel the massive onslaughts by IS, he said, speaking on condition of anonymity.
Footage posted on social media showed military vehicles speeding away from Ramadi, with soldiers hanging off the sides.
Reports said Iraqi forces fled following a series of suicide car bomb attacks on Sunday.
Four almost simultaneous explosions hit police defending the Malaab district in southern Ramadi. Later, three more suicide bombers drove explosive-laden cars into the gate of the provincial military headquarters, the Anbar Operation Command, officials said.
Earlier, Mr Abadi called on pro-government forces to "hold their positions and preserve them and not allow Daesh (IS) to extend to other areas in Ramadi".
"There is continuous air cover that will help ground troops there hold their positions while waiting for support from other forces and the Popular Mobilisation Units," he said, referring to the umbrella group for Shia militias.
The militias played a key role in the government's recent recapture of the city of Tikrit from IS, but pulled out of the city following reports of widespread violence and looting.
The loss of Ramadi represents a very serious setback for the government, and Iraqi officials are alarmed, the BBC's Ahmed Maher reports from Baghdad.
However, the Pentagon said that, while IS had the advantage, there was still fighting in the city.
"We're continuing to monitor reports of tough fighting in Ramadi and the situation remains fluid and contested. It is too early to make definitive statements about the situation on the ground there at this time," said spokeswoman Maureen Schumann, quoted by AFP.
Anbar province covers a vast stretch of the country west from the capital Baghdad to the Syrian border, and contains key roads that link Iraq to both Syria and Jordan.
IS reportedly controls more than half of Anbar's territory.
The deputy head of Anbar council, Faleh al-Issawi, told the BBC that more than 500 people had been killed in the last two days of fighting in and around Ramadi, including policemen who had run out of ammunition, and civilians caught in the crossfire.
Some 8,000 people have been displaced over the same period, according to the International Organisation for Migration.
Troubled history of Anbar province
- Iraq's largest province, which is Sunni-dominated, was occupied by US forces in 2003
- Hostile to the US, fighting quickly broke out between US troops and the region's Sunni insurgents
- The worst battle came in 2004, when thousands died as US troops and coalition forces struggled to take the town of Falluja
- Fighting continued in 2005 and 2006 during which time al-Qaeda in Iraq (AQI) rose to prominence
- The US declared victory in 2007 but AQI remained, resuming attacks in 2011 when US troops withdrew
- Islamic State and other Sunni insurgents currently control much of the province