The Iraqi government imposed a curfew in the western city of Ramadi on Friday over fears that the Islamic State group might try to advance on the strategically important city.

The curfew began at midnight as part of an effort to limit movement in and out of the city as government forces prepared to eliminate pockets of resistance there, said Sabah Karhout, the chairman of the Anbar provincial council. Ramadi, the capital of the vast Sunni-dominated province of Anbar, is located 70 miles west of Baghdad.

The Islamic State group has in recent weeks been making gains against the embattled Iraqi military around Ramadi despite ongoing U.S.-led coalition airstrikes on the militants.

The Islamic State group and allied Sunni militants seized the Anbar city of Fallujah, parts of Ramadi and large rural areas of Anbar early this year. The loss of Fallujah — where American troops engaged in some of the heaviest fighting of the more than eight-year U.S.-led war in the country — foreshadowed the later loss of Iraq's second city of Mosul and much of the north.

Anbar provincial police chief Brig. Gen. Ahmed al-Dulaimi was killed earlier this week while traveling in a convoy north of Ramadi through an area cleared by Iraqi security forces a day earlier, Anbar councilman Faleh al-Issawi said. It was not immediately clear if others were killed or wounded.

The country's Prime Minister Haider al-Abadi met with a delegation from Anbar on Friday, where he urged the province's local tribes to engage with Iraqi security forces in the fight against the Islamic State militants. Iraq's government has repeatedly said that winning over the Sunni tribes is an essential part of the solution.

Ramadi has yet to fall in part because key Sunni tribes in the city have not allowed it to. The Jughaifi and al-Bunimer tribes have helped Iraqi special forces to protect the Haditha Dam in Anbar, and in the battleground town of Dhuluiyah, north of Baghdad, a single tribe, al-Jabbouri, has been the sole resistance to an Islamic State takeover.

In his weekly Friday sermon, Iraq's most revered Shiite cleric, Grand Ayatollah Ali al-Sistani, said Iraqi tribes have always been fundamental to protecting Iraq and its people.

"We are confident that the Iraqi armed forces, the volunteers and the tribesmen in the western areas who are loyal to their country are capable of repelling Daesh attacks and protect their cities ," the reclusive al-Sistani said in a speech delivered by his spokesman Abdul-Mahdi al-Karbalie.

"We urge the Iraq faithful tribesmen — especially those in western Iraq who have been subjected to a fierce campaign by Daesh in recent months — to trust their abilities, and the ability of the Iraqi army to defeat those gangs," he said, referring to the Islamic State group by its Arabic acronym.

Meanwhile, major operations were also underway in Iraq's Salahuddin province to retake key areas from the Sunni militants in between the city of Tikrit, which mostly remains in the control of the Sunni militant group, and the town of Beiji, home to Iraq's largest oil refinery. Two Iraqi military officials, who spoke anonymously because they are not authorized to brief the media, said the military operation was receiving significant aerial support from coalition forces.