Sunni gunmen in Fallujah took advantage of an army pullout earlier this weekThe US Secretary of State has said the US will help Iraq fight al-Qaeda-linked militants, but that it is not planning to send troops back to the country.
John Kerry said he was confident the government of Iraqi PM Nouri Maliki could defeat the militants.
Earlier, the Iraqi government said it had lost control of the strategic city of Fallujah, west of Baghdad.
Al-Qaeda-linked militants now control the south of the city, a security source told the BBC.
Iraqi officials said they have lost control to the militant group, the Islamic State in Iraq and the Levant, known as Isis.
An Iraqi reporter there says tribesmen allied with al-Qaeda hold the rest of Fallujah.
Mr Kerry made the comments as he left Jerusalem for Jordan and Saudi Arabia to discuss his effort to broker peace between Israel and the Palestinians.
"We will stand with the government of Iraq and with others who will push back against their efforts to destabilise," he said.
"We are going to do everything that is possible. I will not go into the details."
He added: "We are not contemplating putting boots on the ground. This is their fight."
The head of the police in Anbar province - Hadi Razeij - said on Saturday that his men had pulled back to the city walls and the people of Fallujah were "the prisoners of Isis".
Fighting there erupted after troops broke up a protest camp by Sunni Arabs in the city of Ramadi on Monday.
They have been accusing the Shia-led government of marginalising the Sunnis.
Local Sunni Arabs have been angered by what they perceive as discrimination by the government of Mr Maliki.
They also say their minority community is being targeted by anti-terrorism measures implemented to stem the surge in sectarian violence.
In recent months, Sunni militants have stepped up attacks across Iraq, while Shia groups began deadly reprisals - raising fears of a return to full-scale sectarian conflict.
On Wednesday, the United Nations said at least 7,818 civilians and 1,050 members of the security forces had been killed in 2013.
The annual death toll was the highest in years, but still significantly below those recorded at the height of the insurgency in 2006 and 2007.
US forces ended combat missions in Iraq in 2010 and left the country in late 2011, having entered in 2003 in a US-led invasion to remove Saddam Hussein from power.
BBC.
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