Dozens of Islamic State fighters struck at dawn, storming government and security compounds in and around the northern Iraqi city of Kirkuk last week, in a coordinated assault more than 100 miles (160 kilometers) from the front lines of the Mosul offensive.

Over the last two years, the extremists have adopted innovative tactics and launched diversionary attacks along the amoeba-like frontiers of their self-styled caliphate, and many now fear they have more surprises in store as Iraqi forces close in on Mosul, the militants' last urban bastion in the country.

The Kirkuk assault was carried out by more than 50 militants who may have been part of so-called sleeper cells. They struck targets in and around the city, pinning down Kurdish security forces for two days and killing at least 80 people. A similar attack was launched on the western town of Rutba, hundreds of miles from Mosul, over the weekend.

Here is a look at some of the other tactics the group may employ.

ATTACKS ON CIVILIANS

As it has suffered a string of battlefield setbacks over the past year, IS has increasingly returned to its roots as a brutal insurgent group, carrying out suicide bombings against civilians, mainly in and around Baghdad.

The group has sought to reassure its supporters that its long twilight struggle will continue, regardless of whether it loses territory. Vastly outnumbered in Mosul, it may respond with attacks on so-called "soft targets" in Iraq or further afield, perhaps seeking to replicate the devastation of the 2015 Paris attacks.

But Iraq is at the greatest risk.

"What happened in Kirkuk might be an introduction to a series of operations, and we cannot rule out the targeting of Baghdad," said Ahmed al-Sharifi, a Baghdad-based military analyst. "There are sleeper cells all over Iraq, particularly in Baghdad."

DIVIDE AND CONQUER

The choice of Kirkuk likely reflected a strategic calculation on the part of IS to sow tensions within the unlikely alliance arrayed against it. The city has long been at the center of a territorial dispute between the central government and the autonomous Kurdish region, where the Mosul operation has seen federal forces deployed for the first time in 25 years.

The Baghdad government and the Kurds are united against IS, but the Kurds have little interest in Mosul, a potentially ungovernable city with a Sunni Arab majority. The Kurds have long prized Kirkuk, however, and could divert their forces, known as the peshmerga, from Mosul to other fronts in order to defend territory they value more.

CHEMICALS AND DRONES

Closer to the front lines, IS may deploy new and unconventional weapons. IS used a homemade drone carrying C-4 explosives to attack French and Kurdish forces in northern Iraq earlier this month, killing two Kurds.

IS is believed to have used crude chemical weapons in both Syria and Iraq, and Iraqi forces have said they are going into battle with protective gear. Last month, an IS rocket containing sulfur-mustard , a chemical agent that causes skin blistering, struck a military base used by hundreds of U.S. troops near Mosul.

No one was wounded in the attack, but Marine Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, called it a "concerning development."

SUICIDE ARTILLERY

Suicide car bombs have featured in Middle East conflicts for decades, but IS might be the first insurgent group to deploy them against conventional forces on the battlefield as a kind of "smart" artillery. The group has already sent more than a dozen armored vehicles loaded with explosives careening toward front-line troops since the Mosul operation began.

Iraqi forces, with the aid of U.S.-led coalition aircraft, have gotten better at blowing them up before they reach their targets, but the weapons still pose a huge risk.

SCORCHED EARTH TACTICS

IS deployed another kind of chemical weapon last week when it torched a sulfur plant south of Kirkuk, sending a cloud of toxic smoke across the Ninevah plain that caused breathing difficulties and nosebleeds up to 30 kilometers (18 miles) away. The fumes mixed with the smoke from oil wells in the region that IS has set alight in recent weeks to try to create a smoke screen.

Many fear that as Iraqi forces converge on Mosul, the extremists could destroy factories, oil installations and other critical infrastructure in a scorched earth campaign. They may also seek to use civilians as human shields. Mosul is still home to more than one million people.

"UNKNOWN UNKNOWNS"

Former U.S. Defense Secretary Donald Rumsfeld famously warned of "unknown unknowns ," things we don't know that we don't know, which somehow captures the challenge posed by evolving militant groups.

The IS capture of Mosul in 2014 — and the fleeing of thousands of Iraqi soldiers and police who were supposed to defend the city in the face of their advance — came as a shock to many people who had never imagined an extremist group could seize a major city. That they have persevered since then, holding onto large swaths of territory despite more than two years of U.S.-led airstrikes and a vast array of forces battling them, also testifies to their dark ingenuity.

"Every time we think we've countered terrorist tactics something new always happens," said David M. Witty, a retired U.S. Army Special Forces colonel and former adviser to Iraqi special operations forces. "There's no end to it."

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Follow Joseph Krauss on Twitter at www.twitter.com/josephkrauss . His work can be found at www.bigstory.ap.org/journalist/joseph-krauss .