ERBIL, IRAQ — The capital of the Kurdish autonomous region was an oasis of calm in an increasingly dangerous sea of chaos Friday.

But there is an agonizing sense of foreboding here over how Iraq’s Kurdish minority might become drawn into the tragedy happening as close as an hour’s drive away in Mosul, where Sunni ultra-fundamentalists from the Islamic State of Iraq and Al-Sham (ISIS) have imposed a draconian interpretation of the Koran amid reports of summary executions.

At least 30,000 young Shiite volunteers responded immediately Friday to an appeal by the country’s leading Shiite cleric, Grand Ayatollah Ali Al-Sistani, to defend Baghdad against the Al-Qaeda-inspired Sunnis, who took several more towns during the day as they advanced on three sides within 80 kilometres of the capital.

Such reports have helped feed a national hysteria that has everyone thinking there will be civil war. Because Iraq has far more people than neighbouring Syria and because trillions of dollars in energy resources are at stake, the conflict is likely to be worse than the Syrian one, in which more than 100,000 people have been killed.

With Iraqi government forces still almost nowhere to be seen, and rival militias vowing mayhem, the prospect of all-out sectarian warfare between the Shiite majority, which now rules the country, and the Sunni minority who had everything its way during the days of Saddam Hussein, has never felt more real or more ominous.

It is almost certain Iraq is going to explode in cataclysmic fighting, with Baghdad and the country’s many oilfields, refineries and pipelines as the prizes.

Once the battle for the capital begins, it is hard to see how Iraq — which was patched together by the French and the British about 100 years ago — will avoid being shattered into Shiite, Sunni and Kurdish fiefdoms. This would, in turn, destabilize an already brittle swath of the Middle East, running from Lebanon through Syria, Jordan and Turkey to Iran.

The Kurds’ formidable militia, the peshmerga, seized control of oil-rich Kirkuk Wednesday, although there were reports Friday the insurgents had started to fight to get it back.

Kurdish news outlets reported ISIS forces were aiming to capture towns along Iraq’s eastern border with Iran. Other reports suggested several hundred Iranian soldiers had crossed into Iraq to support Nouri Al-Maliki, the Shiite prime minister.

While U.S. President Barack Obama said Friday under no circumstances would the U.S. send combat forces back to Iraq, CNN reported USS George H.W. Bush had slipped into the Persian Gulf from the Arabian Sea.

This is naked gunboat diplomacy, but it is hard to imagine how the aircraft carrier’s pilots would be able to find targets. The Sunni extremists have no headquarters, bases or obvious means of communication beyond their ubiquitous cellphones. Rather, they drive around in Mad Max-style pickup trucks, man small checkpoints, and live and travel among the civilian population, creating a situation in which it is difficult to tell friends from foes.

Still, the Kurds have taken in as many as 300,000 refugees from Mosul, a generous gesture considering the tensions in Iraq. Many of them are in Erbil.

Bewildered newcomers spoke Friday about how ISIS forces appeared as if from thin air, while Iraqi police and troops who were supposed to defend them, disappeared into the same thin air.

Kurds have followed the endless blitz of new developments with morbid fascination and a sense of dread. Television newscasts repeatedly played cellphone videos in which zealous Shiites are seen signing up to fight triumphant Sunnis, shown dancing on captured tanks and appearing to execute members of Iraq’s security forces in public.

There were also dire reports Islamic radicals have already imposed their draconian religious beliefs on relatively moderate Mosul. Flyers were distributed ordering women to stay inside unless absolutely necessary; even they leave their homes, they must wear the most conservative Islamic garb. Alcohol and cigarettes have been banned.

Among the Shiite fighters headed for Baghdad are some who were forced out of the Syrian city of Aleppo this spring after several years of savage fighting against Bashar Al-Assad’s regime.

It is astonishing the rebels were able to take Mosul with such ease because no more than 1,000 of them took part in the assault. Still, in a matter of hours they managed to rout a much better-armed and better-equipped U.S.-trained force of 30,000, which conveniently abandoned its weapons and other gear as it disappeared.

That all this happened so quickly and with little actual fighting has lent credence to widespread rumours here Sunni tribal leaders had made a deal with ISIS because they hated Mr. Maliki more than they hated religious zealots.

Curiously, these same tribal leaders worked with American troops only two years ago to try to rid Iraq of Sunni extremism.

In another strange development, the U.S. continues to support Mr. Maliki’s unpopular government, although it is increasingly aligned with Washington’s sworn enemy, Tehran.