Iraqi Defense Ministry spokesman Brig. Gen. Tahsin Ibrahim told CNN that units of the 9th Armored Division had entered the key city and that troops had stormed the neighborhood of al Intisar in the east.
Penetrating the eastern border has been the most significant breakthrough in the offensive, which was launched two weeks ago to free Mosul from the militant group's brutal rule.
Officials had warned that entering the city would likely trigger the fiercest fighting
yet, and that the battle is expected to be fought "street to street," or even "house to house."
• Iraqi security forces shot dead two ISIS suicide bombers in al Intisar, the military says
• Troops combing the neighborhood have detonated at least six car bombs
• Heavy airstrikes and artillery shelling targeted northern Mosul, witnesses say
• Hundreds of civilians poured out as clashes intensified
• Coalition warplanes hit an ISIS convoy, a bridge, an IED factory and two tunnels, US members of the operation say
ISIS leader Al-Baghdadi rallies fighters
About 100,000 forces in an Iraqi-led coalition have taken part in the decisive push toward Mosul, freeing communities from ISIS control
, village by village, along the way. US special forces have supported the operation and continue to take part in airstrikes over the city.
But only Iraqi forces are entering the ground operation in Mosul, commanders say, a testament to Prime Minister Haider al-Abadi's claims that the battle for Mosul is at its core an Iraqi fight, and that sectarian politics must be kept away from the battlefield.
Counterterrorism forces have been on the city's eastern outskirts since Monday but had struggled to push through ISIS snipers, improvised explosive devices (IEDs), suicide attacks and even boulders placed on the main road to the city to slow forces' progress.
Forces made their push into Mosul after ISIS' media arm released audio purportedly featuring the terror group's leader, Abu Bakr al-Baghdadi,
attempting to rally fighters.
Addressing the estimated 5,000 ISIS members holed up in the city, Baghdadi said that "holding your ground in honor is a thousand times better than retreating in disgrace."
Coalition airstrikes, meanwhile, pounded Mosul and hit an ISIS convoy, which included vehicles carrying some senior ISIS members. The convoy was targeted as it crossed the al Khames bridge in the city center, witnesses inside Mosul told CNN. ISIS shut the damaged bridge after the strike.
From the west, the Popular Mobilization Units (PMU) -- a largely Shia paramilitary force assigned the task of advancing west of Mosul -- reported their forces had cut off the main access road used by ISIS to flee to Syria through western Iraq.
Witnesses inside Mosul have told CNN over the past month that ISIS fighters and their relatives have been seen on buses heading out of the city, presumably to the group's de facto capital, the Syrian city of Raqqa.
'We have new life'
As forces broke through Mosul's border, the Iraqi army opened up a safe route out for civilians to evacuate from the front line in al Intisar, the Defense Ministry's Ibrahim told CNN.
A CNN team on the ground saw hundreds of civilians fleeing on foot from the direction of Mosul and the village of Gogjali on the outskirts, risking ISIS' booby traps and gunfire to get themselves and their families out. Many of them waved white flags to show they were civilians.
Hundreds gathered on the side of a road where the Iraqi military sent trucks and buses to pick them up and take them to a camp in the town of Khazir. The men were packed on the back of utility trucks while the women and children were taken by bus.
An elderly woman in a wheelchair who was pushed all the way by a relative to the pickup point said they had traveled for hours after intense fighting overnight.
"We didn't sleep the entire night, and there were rockets. We weren't able to bring our cars through... so we actually walked this entire route," she told CNN.
A young man with her explained that ISIS had placed roadside bombs in front of their homes.
"There are snipers on the rooftops," he said.
Another woman wearing an Islamic niqab -- to hide her face and protect her family members still in Mosul -- described to CNN how ISIS would shoot at civilians as they fled.
"It's very hard to describe our feelings right now. It feels like we have new life breathed into us -- a new soul," she said, holding her young daughter.
Entering Mosul is a significant breakthrough for Iraqi troops.
But this phase of the offensive comes with enormous challenges. Iraqi troops faced strong resistance the closer they got to the city and clearing just the first two blocks of the eastern Mosul neighborhood involved heavy clashes.
One challenge is differentiating ISIS fighters from civilians. The terror group is reported to have brought tens of thousands of civilians -- mostly women and children -- into the city to be used as human shields, a known tactic it uses to ward off airstrikes
and ground assaults.
Inside Mosul, fewer people are attending prayers at mosques as civilians try to stay away from ISIS locations, witnesses said.
Witnesses have told CNN over the past weeks that fighters have rigged bridges with explosives and that suicide squads had been sent in from Raqqa to prepare for the battle. Now they say ISIS teenagers wearing suicide vests are riding around the eastern neighborhoods of the city on motorbikes.
As they go head to head with ISIS in Mosul, coalition forces are pushing from other directions in an attempt to squeeze the militants out.
There were some signs Thursday the efforts were working.
Witnesses said airstrikes over the past few days had forced ISIS to largely abandon checkpoints inside the city, manning them only at night.