The Iraqi Prime Minister Nouri al-Maliki vowed Saturday to eliminate "all terrorist groups" from Anbar province as a security source conceded the government had lost control of Fallujah to Al-Qaeda-linked fighters.

Maliki, speaking on state television, said his government would end "fitna," or disunity, in the province and would "not back down until we end all terrorist groups and save our people in Anbar."

The overrunning of Fallujah and Ramadi this week by the group Islamic State of Iraq and Greater Syria (ISIS) in the Sunni heartland of western Anbar provinces is a blow to the Shia-led government of Malik. His government has been struggling to contain discontent among the Sunni minority over Shia political domination that has flared into increased violence for the past year.

On Friday, ISIS gunmen sought to win over the population in Fallujah, one of the cities they swept into on Wednesday. A commander appeared among worshippers holding Friday prayers in the main city street, proclaiming that his fighters were there to defend Sunnis from the government, one resident said.

"We are your brothers from the Islamic State in Iraq and Levant," gunmen circulating through the city in a stolen police car proclaimed through a loudspeaker. "We are here to protect you from the government. We call on you to cooperate with us."

At least 40 of the ISIS fighters, who fought with machine guns and pick-up trucks mounted with anti-aircraft guns, were killed this week in Ramadi, medical and tribal sources told Reuters. There was no casualty figure for tribesmen or security forces. On Friday, two policemen were killed and six other wounded when their patrol was attacked by gunmen in speeding cars outside Fallujah, a police officer and a medical officials said.

The fighters seized the moment after Maliki ordered security forces to break up a yearlong sit-in near Ramadi. There, Sunnis had gathered to protest their exclusion from the political process by the Shia-led central government, but Maliki claimed the protest became a camp for Al-Qaeda.

Once the sit-in was broken up on Monday, fighting erupted between the security forces and local fighters — among them, elements of ISIS, which have long battled for dominance in Ramadi and other Anbar cities.

On Thursday, tribesmen angry at what they perceive as Sunni marginalization in politics clashed with Iraqi troops trying to regain control of Falluja and Ramadi.

But later that day, the tribesmen struck a deal with the government to join forces against ISIS fighter seeking to establish local control.

"Those people are criminals who want to take over the city and kill the community," said Sheikh Rafe'a Abdulkareem Albu Fahad, who is leading the tribal fight against ISIS in Ramadi.

Anbar — Iraq's largest province — is composed of a vast desert area on the borders with Syria and Jordan and has an almost entirely Sunni population. It was the heartland of the Sunni insurgency that rose up against American troops and the Iraqi government after the 2003 U.S.-led invasion that toppled Saddam Hussein.

The insurgency was fueled by anger over the dislodgment of their community from power during Saddam's rule and the rise of Shias. It was then that Al-Qaeda established its branch in the country.

Anbar became the deadliest territory for coalition troops, with fevered local opposition to the U.S. culminating in the lynching of four American contractors in Fallujah in 2004. This set off two U.S. military offensives that year to take back the city.

Responding to the latest developments, Marie Harf, deputy spokesperson for the State Department, said Washington was following the events in Anbar closely.

"(ISIS's) barbarism against civilians of Ramadi and Fallujah and against Iraqi Security Forces is on display for all to see," Harf said in a statement.

"We would note that a number of tribal leaders in Iraq have declared an open revolt against ISIL. We are working with the Iraqi government to support those tribes in every possible way."