Until now, apparently.
Heavily armed militants attacked an abandoned hotel in central Kirkuk that local police had used as their headquarters.
Peshmerga and Kurdish anti-terror units later raided the hotel, wresting control of it from the militants and killing three of them, according to Peshmerga sources. In addition, two suicide bombers detonated themselves in an attempt to keep the Kurdish forces out.
Also Friday, ISIS militants took over Maktab Khalid, an area about 12 miles southwest of Kirkuk, after heavy clashes with the Peshmerga.
Among those killed was Brig. Gen. Shirko Fateh, the highest-ranking operational commander of the Peshmerga brigade located in Kirkuk.
Photos posted by ISIS purportedly show the group's militants in control of parts of south and southwest Kirkuk, burning tents that had been used by Peshmerga troops.
Chemical weapons expert killed, U.S. says
The U.S. military said Friday that an ISIS chemical weapons expert was killed during a coalition strike late last week.
Abu Malik worked in Saddam Hussein's chemical weapons program before joining al Qaeda in 2005, U.S. Central Command said.
He was killed January 24 near Mosul.
"His death is expected to temporarily degrade and disrupt the terrorist network and diminish (ISIS') ability to potentially produce and use chemical weapons against innocent people," the military said.
There is no public evidence that ISIS has a dedicated weapons of mass destruction program.
But U.S. Central Command said: "His past training and experience provided the terrorist group with expertise to pursue a chemical weapons capability."
Oil reserves make Kirkuk a big prize
Kirkuk is a strategically important city in the months-long fight, one that has pitted ISIS against the Peshmerga, Iraqi government troops and an international coalition that has carried out airstrikes against the terrorist group.
It is one of the few notable cities -- apart from the region of Kurdistan and its capital, Irbil -- in northern Iraq that haven't fallen to ISIS. Part of its significance stems from the fact its oil reserves are almost as much as those in southern Iraq.
The Kurds and the central Iraqi government in Baghdad have long wrangled over control of those reserves, with each side wanting to keep hold of them. ISIS, which relies heavily on revenue from oil smuggling to fund its operations, has been coveting them, too.
Peshmerga forces took over the Kirkuk area in June when the Iraqi army crumbled in the face of ISIS' advances and have played a vital role in defending it from ISIS since.
In December, ISIS claimed responsibility for a suicide car bomb attack there that killed at least 17 people and injured more than 20. The attack, according to ISIS, was meant to send a message to the Kurdish people and Peshmerga fighters.
Still, Kirkuk is hardly the only place that has seen recent fighting -- which may be part of ISIS' rationale for Friday's attack there.
The group has been fending off an offensive from Peshmerga fighters around Mosul, Iraq's second-largest city and a focal point for all sides in the conflict, that has left the Sunni extremist group
on its heels.
The city of 1.5 million people on the Tigris River has been held by ISIS since June. ISIS has invested heavily in governing the city. Its leader, Abu Bakr al-Baghdadi, pronounced his leadership of the caliphate at the Grand Mosque there in July.
Kurdish officials say that as long as ISIS holds Mosul, it threatens Kurdistan. Likewise, neither the government in Baghdad nor its coalition partners can rest while terrorists occupy Iraq's second-largest city.
Peshmerga forces have made steady progress against ISIS north and west of Mosul over the past two months.
They have taken some 3,000 square kilometers (1,160 square miles) of the Sinjar area, as well as the area around the Mosul Dam, choking off access routes and threatening ISIS' main resupply routes.
Officials: Iraqi forces fend off Ramadi attack
There's little doubt, though, that ISIS remains a very real force, and threat, in much of Iraq.
The group, which calls itself the Islamic State, still controls a vast swath of that Middle Eastern nation and neighboring Syria. Its goal is to have a vast caliphate under its strict version of Sharia law, with its followers proving they will stop at nothing -- having been blamed for the large-scale killings of civilians, mass kidnappings and forcing women and girls to become sex slaves -- during its quest.
That violent campaign continued Friday, and not just in Kirkuk.
Dozens of gunmen believed to be from ISIS faced off Friday morning about 175 miles (285 kilometers) away in central Ramadi, police and health officials in that city said.
Several hours later, that onslaught had been foiled and 20 gunmen were dead, according to the officials.
Elsewhere in Ramadi, a suicide car bomb explosion at an Iraqi army checkpoint killed one soldier and wounded six others.
Violence flared in other parts of Iraq as well that, while it hasn't been tied to ISIS, is further proof of the country's unsettled state.
Six explosions went off Friday around Baghdad, leaving at least seven dead and 23 wounded, according to police officials.
The deadliest such blast was in Bab Al Sharji, a busy commercial area in central Baghdad, leaving three dead and 10 hurt.