BAGHDAD — The Iraqi military backed by US-led coalition aircraft launched a long-awaited operation Thursday to recapture the northern city of Mosul from Islamic State militants, a military spokesman said.

In the push, Iraqi forces retook several villages on the outskirts of the town of Makhmour, east of Mosul, early Thursday and hoisted the Iraqi flag there, according to the spokesman for the Joint Military Command, Brigadier General Yahya Rasool.

Mosul has been under the control of the Islamic State since the militants’ surprise offensive in spring 2014. US and Iraqi officials have cast the retaking of Mosul as a symbolically and politically crucial step for the Iraqi government.

It was not immediately clear how long such a complex and taxing offensive would take.

Only recently, Iraqi and US officials refrained to give a specific time on when the Mosul operation could begin, saying it would take many months to prepare Iraq’s military for the long-anticipated task of retaking the key city.

Some US and Iraqi officials have said it may not even be possible to retake it this year, despite repeated vows by Prime Minister Haider al-Abadi.

Iraqi state-run TV interrupted its morning program Thursday with a series of news alerts announcing the operation in Ninevah province and broadcasting patriotic songs and flag-waving video clips.

Rasool said the US-led international coalition was providing air support but would not divulge more details on the offensive, which he said was dubbed ‘‘Operation Conquest.’’

Mosul — Iraq’s second-largest city — fell to the Islamic State during the militants’ June 2014 onslaught that captured large swaths of northern and western Iraq and also neighboring Syria. Mosul, about 225 miles northwest of Baghdad, became the largest city in the Islamic State’s self-declared caliphate on the territories the militants control.

Rasool’s declaration came only few days after the United States announced that it has set up a small Marine artillery outpost in northern Iraq to protect a nearby Iraqi military base in Makhmour — the likely staging ground for a Mosul assault, 40 miles southeast of the city.

On Saturday, the militants fired two rockets at the base, killing a US Marine and wounding several others.

Despite Thursday’s announcement, the number of Iraqi troops needed to carry out the operation to retake Mosul nearly two years after it fell to ISIS are not yet in place and training efforts by the US-led coalition are ongoing.

Under political pressure to show victory, Abadi has repeatedly vowed to liberate Mosul but Lieutenant General Vincent Stewart of the United States, head of the Defense Intelligence Agency, told Congress last month that he is ‘‘not as optimistic.’’

The Iraqi military must also clear Islamic State fighters from more than 70 miles of territory to ensure reliable supply lines between Makhmour and Baghdad.

One leg of the Iraqi military’s efforts to clear some of that territory in Anbar has been put on hold. A political crisis in Baghdad has prompted Abadi to pull some of Iraq’s elite counterterrorism forces back from the front in the Euphrates River valley to secure the capital.

The prime minister recalled the forces after influential Shi’ite cleric Moqtada al-Sadr mobilized thousands and staged a sit-in outside Baghdad’s highly fortified green zone last week in a show of force meant to put pressure on Iraq’s political leadership.

Kurdish and US military officials were more cautious Thursday in describing the operation near Makhmour as the start of the invasion of Mosul.

Iraqi officials have previously announced military offensives against the Islamic State that were not matched by successes on the ground. Several times last year, officials said they had begun the liberation of Ramadi, the capital of Anbar province, but the city was not retaken until the end of the year.