Sinjar, Iraq (CNN) Plumes of smoke blackened the sky above Sinjar as Kurdish forces, backed by intense coalition air support, tried Thursday to take back the northern Iraqi town from ISIS .

The operation includes up to 7,500 Peshmergas -- the Kurdish military force -- who are attacking the city from three sides to take control of supply routes, according to the Kurdish Region Security Council.

CNN senior international correspondent Nick Paton Walsh is with one of the three fronts of fighters who launched their liberation operation early Thursday morning against a backdrop of airstrikes.

The U.S.-backed coalition Operation Inherent Resolve said coalition aircraft have conducted more than 250 airstrikes across northern Iraq in the last month. The strikes have reportedly destroyed ISIS fighting positions, command and control facilities, weapon storage facilities, improvised explosive device factories, and staging areas.

"A pitch-black sky was lit up by a lot of coalition airstrikes following days of bombing. At dawn, a large procession of Peshmerga started snaking their way through Sinjar mountain and behind it," Paton Walsh said.

The coalition strikes were pounding the strategic city itself, he said, with four different columns of smoke darkening the horizon above: "The strikes on Sinjar almost make the sky over it look black. There's a vast amount of air power -- more intense than the fight for Kobani."

JUST WATCHED Liberation from ISIS comes at a cost Replay More Videos ... MUST WATCH Liberation from ISIS comes at a cost 02:19

Kobani is a Syrian border town that was wrested back from ISIS militants earlier this year after four months of fierce fighting that left parts of it entirely flattened.

According to a Pentagon spokesman, U.S. troops are in the field calling in airstrikes from positions in Sinjar.

"The Peshmerga forces are carrying this out with, as you said, the support of coalition advisers. There are U.S. personnel. My understanding is there are coalition advisers from other countries as well participating," Pentagon spokesman Peter Cook told reporters.

He added: "Most of those folks as I understand it are behind the front lines advising and working directly with Peshmerga commanders. There are some advisers who are on Sinjar mountain assisting in the selection of airstrike targets."

Late Thursday, the Kurdish Region Security Council released a statement detailing the day's operations. It reported that more than 150 square kilometers (60 square miles) had been retaken from ISIS.

"Peshmerga units will continue from three fronts to set up defensive positions, allowing engineering teams to clear the heavily mined area. Peshmerga forces have already achieved two of three strategic goals, dealing a significant blow to ISIL morals," it read, using an alternative acronym for ISIS. "The final objective -- to enter and clear the city -- will be underway soon."

Peshmerga and coalition unity

Reclaiming Sinjar is one big step toward dividing the "caliphate" that ISIS claims it is establishing across the region.

The artery that passes through the town links the Iraqi city of Mosul -- ISIS' prized possession -- with cities it holds in Syria.

Paton Walsh said the highway was a key goal for the Kurdish fighters, who were equipped with vehicles ranging from pickup trucks to armored Humvees.

"One of the targets of this offensive is the highway that runs through Sinjar, known as Route No. 47 to many. Now that's very important, not only of course because of what it does to liberate the population of Sinjar -- those who've not fled ISIS rule having endured it now for over a year -- but also because it is a vital supply route towards Mosul, another key target of any future coalition offensive," he said.

About 1.5 million people still live in Mosul, where prices are rising and activists report hunger.

The U.S.-backed coalition said "Operation Free Sinjar" was aimed at clearing ISIS from Sinjar and seizing portions of Highway 47.

"By controlling Highway 47, which is used by Da'ish to transport weapons, fighters, illicit oil, and other commodities that fund their operations, the Coalition intends to increase pressure on Da'ish and isolate their components from each other," it said in a statement. Da'ish is the Arabic acronym for ISIS.

JUST WATCHED ISIS hates this religious group the most Replay More Videos ... MUST WATCH ISIS hates this religious group the most 01:57

"This operation will degrade Da'ish's resupply efforts, disrupt funding to the terrorist group's operations, stem the flow of Da'ish fighters into Iraq, and further isolate Mosul from Ar Raqqah," said coalition spokesman Col. Christopher C. Garver. The Syrian city of Ar Raqqah, also spelled Raqqa, is ISIS' de facto capital.

By Thursday afternoon, the Kurdish fighters pushing toward Sinjar had taken control of a number of villages near the Iraqi town.

"Along that highway there's one village, Kabara, that's been repeatedly hammered by airstrikes in the past hour or so and a lot of Kurdish forces have managed to move into the main road," Paton Walsh said. Tweets by Kurdish fighters showed that almost all the vehicles in the village had been "burned to a crisp."

A tough slog

Before the push to retake Sinjar began, Kurdish fighters said they knew it wouldn't be easy.

Peshmerga commanders estimate some 600 ISIS fighters are inside Sinjar, with recent reinforcements boosting the militants' numbers. The Kurdish fighters believe they will encounter hundreds of landmines and booby traps.

Paton Walsh said it was unclear how ISIS would respond to the offensive.

"As you've seen in the past, sometimes ISIS have decided that certain fights are not worth them staying for the long haul, and I think there is a certain amount of manpower and mass here -- and also coalition air power, which we heard from the top of Mount Sinjar, during a very dark, cold night yesterday, pound targets consistently around that particular city."

Paton Walsh said there had been a "substantial uptick" in airstrikes on Sinjar in the days leading up to the launch of the offensive.

Speaking to CNN's Fareed Zakaria, U.S. Secretary of State John Kerry said he's confident Sinjar will fall, but was reluctant to say when.

"But I am convinced that Sinjar will be liberated, as we have liberated Tikrit. And currently the Iraqi forces are moving on Ramadi," he said.

Kerry explained the strategy.

"President Obama, at the very beginning, said we're going to degrade and defeat ISIL. We're going to stabilize the countries in the region -- Jordan, Lebanon, work with Turkey -- and we are going to seek a political settlement," he said. "That is exactly the strategy today and it is working -- to a degree -- not as fast as we would like, perhaps, but we are making gains."

How will ISIS respond?

"I think the issue will be for ISIS, given the nature of the offensive -- from three different directions -- quite what their best strategy is: to sit here and try and symbolically hold it as long as they can, or pull out," Paton Walsh said.

"ISIS of course may also be feeling pressure on other fronts. There's been a lot of talk about the possibility of a move against Ramadi for the past few months.

JUST WATCHED They survived ISIS, but may not survive the winter Replay More Videos ... MUST WATCH They survived ISIS, but may not survive the winter 02:47

"We've not seen any evidence of that at this particular stage but there is a genuine feeling that maybe the coalition -- after months of paralysis, months of calm -- might also slowly be beginning to get some kind of harmony or synchronicity here in terms of moving on separate fronts against ISIS and perhaps stretching what resources they have a little bit thinner."

A coalition spokesman in Baghdad told reporters later Thursday that Iraqi security forces had begun to encircle Ramadi, with support from coalition air power.

ISIS fighters swept into Ramadi in May, tightening control of Iraq's Anbar province and gaining a base of operations about 110 kilometers (70 miles) away from the capital, Baghdad.

Paton Walsh said the operation to retake Sinjar was important symbolically.

"The Peshmerga here want to show that they can be united with coalition air power, with Western military advisers, who we understand are in their midst here as well, to launch a successful -- and they hope brief -- offensive towards this town, but also strategically, because of what Sinjar could mean in the future, down the line."

Photos: Agony of the Yazidis Photos: Agony of the Yazidis A Yazidi woman kisses the hand of a relative before a bus takes women and children who were captives of ISIS to an airport in the Kurdish region of Iraq. From there, they will fly to Germany, where the German government is resettling up to 1,000 former captives of ISIS, giving them housing and psychological treatment. Hide Caption 1 of 8 Photos: Agony of the Yazidis Sabah Mirza Mahmoud, right, cries next to his uncle Jamil Jato as Mahmoud and his sisters prepare to leave for Germany. Hide Caption 2 of 8 Photos: Agony of the Yazidis Baba Sheikh, the Yazidi spiritual leader, speaks to Yazidi women and girls who were captives of ISIS before their journey to Germany. Hide Caption 3 of 8 Photos: Agony of the Yazidis A Yazidi girl faints while saying goodbye to relatives who are going to Germany. Hide Caption 4 of 8 Photos: Agony of the Yazidis Jamil Jato shows photos of family members who were murdered by ISIS. Hide Caption 5 of 8 Photos: Agony of the Yazidis Two older Yazidi men talk under a tree. Hide Caption 6 of 8 Photos: Agony of the Yazidis Women and children wave goodbye to relatives before their journey to Germany. Hide Caption 7 of 8 Photos: Agony of the Yazidis A Yazidi man lights a ritual oil candle at sunset. Hide Caption 8 of 8

He said the Kurdish fighters appeared optimistic they would take back Sinjar.

"I think the hope amongst the Peshmerga and the coalition is that the level of manpower they have here, their dominance in the skies, means potentially this could be over in days," he said. "But with a town of this size which had tens of thousands living in it before -- which ISIS has had months to prepare for an onslaught against -- this could turn out to be trickier than some are hoping."

Retired Lt. Col. Rick Francona, a CNN military analyst, agreed that the fight in Sinjar would be slow going.

"They're going to have to slog through this house by house, street by street," he said. "It's going to be very difficult."

More than a year under ISIS

JUST WATCHED From 2014: Dramatic rescue as Yazidis flee ISIS Replay More Videos ... MUST WATCH From 2014: Dramatic rescue as Yazidis flee ISIS 02:10

The world watched in horror last year as some 50,000 Yazidis, who live in the region, scrambled up Mount Sinjar to escape the ISIS onslaught. About 5,000 men and boys in Sinjar and nearby villages were massacred, according to U.N. estimates, while teenage girls and women were sold into slavery

Since then, Sinjar has become a chaotic jumble of demolished buildings held by ISIS fighters.

"There is no reliable estimate as to how many civilians still live inside of Sinjar," Paton Walsh said.

"You can tell how many seem to have fled, from the tents the Yazidis have erected up around Mount Sinjar -- even in this bitter cold -- still enduring a life here, wanting to be near their hometown. But that is the key concern obviously in situations like this. Many will be fearing that the amount of lead-up time has given ISIS adequate ability to ensure the civilian population are in place to assist them in protecting themselves."

The Peshmerga said they wanted to establish a buffer zone to protect the civilian population, but it was not entirely clear how that would physically work, Paton Walsh said.

Neighbors unite

With the operation to retake the town looming, some 5,000 Yazidi fighters were mobilized under the command of the Kurdish Peshmerga. Most are farmers; a very few have military experience.

The Yazidis are one of the world's smallest and oldest monotheistic religious minorities. Their religion is considered a pre-Islamic sect that draws from Christianity, Judaism and the ancient monotheistic religion of Zoroastrianism. In ISIS' eyes, they're infidels.

The Yazidis and Kurds have lived side by side for thousands of years and are friendly neighbors.

The Kurds are Sunni Muslims, who have their own unique language and culture. They occupy an autonomous region in northern Iraq, but the Kurdish homeland also covers portions of Iran, Turkey, Armenia and Syria.