Rebels and Iraqi police in the volatile Anbar province say the Iraqi government has lost control of the city of Fallujah to al-Qaida militants after days of fighting.
The al-Qaida-linked Islamic State of Iraq and Syria, which is active in both countries, has pushed police out of the city center.
Iraqi Prime Minister Nouri al-Maliki said the country's forces will not retreat from Anbar province until they "eliminate" al-Qaida militants there.
''The people of Anbar province are now standing shoulder to shoulder with the Iraqi armed forces. This is the true stance of Iraqis. They (the people of Anbar province) are once again carrying weapons to chase al-Qaida members. This is the real attitude of Iraqis.''
On Friday, al-Qaida militants raised their flag over government buildings in Fallujah, about 60 kilometers west of Baghdad, and declared an independent Islamic state.
Witnesses said the militants cut power lines in the city late Friday and ordered residents not to use backup generators.
The State Department said Saturday ISIS was displaying "barbarism against civilians" in Fallujah and the Anbar capital of Ramadi. Deputy spokesperson Marie Harf noted the United States was working to support "in every possible way" several tribes that have revolted against ISIS.
A local journalist who asked for anonymity out of fear of retribution told The Washington Post that police and other government-aligned forces had abandoned Fallujah and that al-Qaida had burned all Iraqi national flags.
Fighting across the vast open spaces of western Iraq has become a severe test of Prime Minister Nouri al-Maliki's ability to hold the country together and prevent full-scale civil war.
A tribal leader in Ramadi who fought alongside U.S. troops in 2007 told The Washington Post his fighters had joined police in ejecting al-Qaida loyalists.
The explosion of violence in western Iraq is pitting al-Qaida-linked Sunni extremists, who now control large swaths of the region west of Baghdad, against forces of the Shi'ite-dominated central government. Government forces in the west are backed by local tribesmen who have chosen to align themselves with Baghdad rather than with al-Qaida-linked fighters.
Anbar province was the center of the Sunni insurgency during the eight-year presence of U.S. military forces, which withdrew from the country in December 2011. More than 1,300 U.S. military personnel were killed in the region.