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Iraqi security forces stood guard on Friday at the Baiji refinery north of Baghdad.Credit Thaier Al-Sudani/Reuters

BAGHDAD — Iraqi forces and the Shiite militias fighting alongside them announced Friday that they had retaken the oil refinery at Baiji from Islamic State militants, in some of the first significant progress against the extremist group after months of stalled efforts.

“Baiji refinery has been completely liberated from Daesh,” a spokesman for Iraq’s counterterrorism service said, using the Arabic acronym for the Islamic State, which is also known as ISIS or ISIL. “Removing the I.E.D.s will take some time, but we can say that we liberated the whole of the refinery,” he added, referring to improvised bombs.The Baiji refinery has been damaged so severely that it could take years to get it functioning again. But Iraqi officials insist that its recapture is a strategically important step, and a vital lift to morale, in the broader campaign against the Islamic State, which controls much of northern and western Iraq.Baiji and the nearby town of Siniya, which Iraqi forces said they had also taken, are on a major north-south route to Mosul, Iraq’s second-largest city, which was seized by the Islamic State in June 2014.Continue reading the main story Graphic Where ISIS Has Directed and Inspired Attacks Around the World

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“This battle is crucial,” Prime Minister Haider al-Abadi said Monday during a visit to Salahuddin, the province north of Baghdad that includes Baiji.

Islamic State militants and Iraqi forces have been fighting for more than a year over Baiji, which has changed hands repeatedly. The difficulties the Iraqis have had in holding onto the refinery point to the more formidable challenge they would confront in any operation to retake Mosul or other Islamic State strongholds.

The Iraqis appeared to have had more going for them this time. The push to retake Baiji came as Iraqi forces were mounting a parallel offensive to retake Ramadi, the capital of Anbar Province in western Iraq.

The Iraqi military said it had encircled Ramadi, which United States military officials say is defended by an estimated 600 to 1,000 Islamic State fighters. Even though Iraqi troops have yet to enter the city, the simultaneous operations are pressuring the Islamic State’s forces.

The Shiite militias — formally known as the Popular Mobilization Forces — have not been given a role in the Ramadi operation for fear that their presence might antagonize the mostly Sunni population there. But the militias, some of which are backed and trained by Iran, played a major role in the Baiji operation.

That offensive is being overseen by Maj. Gen. Juma al-Jubouri, who runs the military command center in Salahuddin. The American-led coalition, which has supported the offensive with airstrikes, said Iraq’s counterterrorism service and federal police had led the attack on Baiji.

A spokesman for Shiite militias said that several thousand Shiite militiamen were fighting in and near Baiji, which is more than the estimated number of Iraqi soldiers also fighting there. According to figures provided by Iraqi police spokesmen, 1,700 Iraqi national policemen and 1,200 local policemen were involved in the operation.

Shiite militia leaders have advertised their presence on the battlefield. Qais al-Khazali, the head of the Asaib Ahl al-Haq, a militia long supported by Iran, was filmed inside the captured refinery. Hadi al-Ameri, the leader of the Badr Organization, another Iranian-backed group, also played a visible role in the operation.

In the southern city of Karbala, a representative of Grand Ayatollah Ali al-Sistani, the spiritual leader of Iraq’s Shiites, said during his Friday Prayer sermon that the fight against the Islamic State was among “the holiest battles in our contemporary history.”

The representative, Ahmed al-Safi, urged Iraqi forces to respect human rights and not to steal from residents, a noteworthy injunction after accusations of serious abuses by some militias.

“Make sure to be just and righteous in all of your steps,” he said. “Protect the elderly, the women, children and any innocent person.”

The next step after Baiji, Iraqi officials said, will include operations to take the nearby towns of Hawija and Shirgat, where tough fighting is expected. The Iraqi military said its Sukhoi SU-25 attack jets had carried out airstrikes in Shirgat and on an Islamic State convoy on the road from Mosul.

The American-led coalition said that it had carried out numerous airstrikes at Baiji to weaken the militants as the Iraqi forces moved in. A coalition spokesman said Thursday that there had been 43 airstrikes in Baiji over the past 30 days.