Last Updated Nov 12, 2015 6:12 PM EST
SINJAR, Iraq -- Kurdish Iraqi fighters backed by U.S.-led airstrikes launched an assault Thursday to retake the key town of Sinjar, which Islamic State of Iraq and Syria (ISIS) militants overran in August 2014.
The militants massacred thousands of minority Yazidi men in the city and raped and abducted thousands of women. Their seige forced tens of thousands of Yazidis to flee into the mountains, and drew the first U.S. airstrikes against ISIS.
The Kurdistan Region Security Council (KRSC) tweeted a claim of significant progress early on in "Operation Free Sinjar," saying Peshmerga had gained control of stretches of a key highway both to the east and west of the city.
Late Thursday night, the council said in a statement that reports indicated 100 ISIS militants had been killed and nearly 60 square miles of ISIS territory had been retaken during the day.
There were no reports of Kurdish casualties.
CBS News correspondent Charlie D'Agata was embedded with Peshmerga fighters as they approached the city. He said smoke filled the air over Sinjar all night and through the morning amid intense airstrikes by U.S. and coalition aircraft, hammering suspected ISIS targets in and around the city.
At dawn, D'Agata watched as thousands of Peshmerga wound down from their position on a mountainside toward the ISIS front lines. They filled every conceivable type of vehicle, from Humvees and MRAPs to pickup trucks with mounted machine guns to ordinary civilian cars.
Peshmerga officials said 7,500 troops and volunteers were taking part in the massive ground offensive. They were pushing in from three sides and they claimed early success in both the east and west of the city.
One U.S. military official told CBS News they believed the Peshmerga would be able to retake Sinjar in "2-4 days... and another week to finalize clearing operations."
U.S. Army Col. Steve Warren, spokesman for the U.S. mission against ISIS, told CBS Radio News American advisers on the ground were working with Kurdish "spotters" to help develop targets. He said there had been U.S. advisers in the region for some time, but additional Americans were sent to the area in the past week or so to assist in this operation.
"All of the U.S. advisers are staying well away from the very front lines, not in a position where they will encounter any direct contact with the enemy," Warren said.
The key is to take and hold territory along Highway 47, the main supply route between ISIS' headquarters of Raqaa in Syria and their stronghold of Mosul in Iraq.
D'Agata notes that Sinjar also sits squarely in Kurdish territory, and the Kurds want it back.
But it was slow going. Col. Warren told CBS News the U.S. had intercepted ISIS communications in Sinjar ordering fighters to "stand to the end," and American commanders expected it to be a tough fight.
Even backed by the U.S. airstrikes and with Peshmerga artillery raining down on the city, the Kurdish fighters said they faced threats from snipers, booby trap bombs on the roads and in the buildings, and suicide bombers driving truck bombs straight into them as they advance.
"(Peshmerga) troops are holding their position, waiting for reinforcements and more airstrikes so they can then move into the center of the town. Airstrikes have been very important to the operation getting to the point where it is now," said Maj. Gen. Seme Busal, one of the front line Peshmerga commanders.
ISIS has used the Highway 47 corridor for months to ferry fighters, weapons and other supplies back and forth across the border.
"If you take out this major road, that is going to slow down the movement of (ISIS' quick reaction force) elements," Capt. Chance McCraw, a military intelligence officer with the U.S. coalition, said earlier in the week.
Warplanes in the U.S.-led coalition have been conducting strikes around Sinjar for weeks, carrying out more than 250 airstrikes since the end of September alone. D'Agata said he and his team had lost count of the number of strikes carried out overnight from Wednesday into Thursday.
On Thursday afternoon, the Pentagon said the coalition launched 36 airstrikes in support of the operation, CBS Radio News' Cami McCormick reports. According to a Pentagon official, command centers in Erbil, Iraq, and the country of Qatar process information coming from the U.S. advisers and the Peshmerga and then select the targets for the strikes.
The ISIS militants were burning tires in Sinjar in an apparent effort to sheild their activities from aircraft. A U.S. military spokesman confirmed the tire burning, but told McCormick, "the smoke doesn't bother us."
There were an estimated 600 ISIS fighters in the town as Operation Free Sinjar began. The militant group was also thought to have been ramping up its presence in advance of the anticipated Peshmerga offensive.
Sinjar is on the northern edge of ISIS' territory, at the foot of Sinjar Mountain and about 30 miles from the Syrian border, and it's not an easy target. A previous attempt by the Kurds to retake it stalled in December.
Sinjar was captured by ISIS around the beginning of August 2014, just after the extremists seized Mosul in their devastating blitz across northern Iraq.
The Sunni Muslim terror group unleashed a wave of terror against the minority Yazidis, an ancient Shiite Muslim sect that ISIS views as heretics and devil worshipers.
Some 40,000 Yazidis fled up Mount Sinjar and into the surrounding hills as the militants surrounded them. Video from the top of the mountain showed huge numbers of refugees living on the rocky peaks, exposed to the desert sun and with only the clothes on their backs -- no food or water.
The ISIS siege against the civilian population prompted the U.S. and its allies to quickly drop food and water onto the top of Mount Sinjar, and that was followed about a week later, on Aug. 9, by airstrikes against the militants.
Eventually the strikes cleared a path for the refugees to escape from the mountain, but the success was limited and shortlived. ISIS advanced on Sinjar again just days later.