The battle for Fallujah is shaping up to be unlike any of the other assaults in the Iraqi military's town-by-town war with the Islamic State group.

In the nearly two weeks since the operation began, airstrikes have been used sparingly, Shiite militias have so far been kept to the perimeter, and the initial advance on the symbolically important town has been slow.

U.S.-trained Iraqi counterterrorism forces, wary of coming street battles in the city, are already facing fierce resistance on the outskirts from well-entrenched militants. Those fighters are believed to include many foreign jihadis who are considered better-trained that the ones in towns that have been retaken in recent months.

In Ramadi — the last major victory for Iraqi forces against IS — many of the militants were able to flee to other strongholds along the Euphrates River valley. Now, all of that territory has been cleared, and the extremists have no escape route from Fallujah.

That suggests a long fight for the city less than an hour's drive west of Baghdad.

While Fallujah is smaller in area than Ramadi, an estimated 50,000 people are trapped in the city, twice as many as were in Ramadi when it was recaptured.

Aid groups say about 1,000 families have managed to flee the outskirts of Fallujah since the operation began May 22. But the Norwegian Refugee Council, an international humanitarian group that does extensive work in Iraq's Anbar province, says none of the civilians trapped in the center of the city have made it out.

Residents have told The Associated Press that IS fighters tightly control all roads in and out of the city and have threatened to kill anyone who tries to escape.

"There is a clear difference between the Fallujah and Ramadi operations," said Iraqi military Brig. Haider al-Obeidi.

The terrain is a challenge, he said, because the outskirts are dotted with orchards and irrigation canals that give IS fighters working in small mobile units an advantage over the slow-moving convoys of armored vehicles

The fighters his men are encountering are better trained than those in pervious battles, al-Obeidi said.

Military commanders are reporting larger numbers of foreign fighters in Fallujah.

"Their snipers are smart, they hit essential parts of the bulldozers' engines and Humvees' wheels," he said. Repairing them takes time and slows advances, and the forces have not received any new weaponry or additional training that would help, al-Obeidi said.

The troops have few options beyond trying to protect their units from IS counterattacks, he said. As streets are cleared, they plan to erect roadblocks to guard against suicide car bombs, one of the deadliest IS tactics.

Iraqi forces are continuing to move forward, snaking through the desert on Fallujah's southern edge. Smoke rose Friday from a cluster of factories and industrial buildings in the area. On the city's northeastern edge, Shiite militia forces fired mortar rounds into the city from suburbs cleared by Iraqi federal police.

Another big concern in the assault is the tension between Sunnis and Shiites.

Fallujah, part of the Sunni heartland of western Iraq, has long been a bastion of bitterness toward the Shiite-led central government in Baghdad that emerged after the 2003 U.S. invasion that toppled Saddam Hussein.

The city was the heart of the Sunni insurgency against U.S. forces in the mid-2000s that eventually gave birth to al-Qaida in Iraq. As revolution swept across the Middle East in 2011, Iraq's own anti-government protest movement began in Fallujah and quickly mobilized millions across the Sunni-majority provinces. The Islamic State group took it over in 2014.

Military commanders say the mostly Shiite militias, formally incorporated into the government's Popular Mobilization Forces umbrella group, will not be part of any push into the center of the city. The militias have been accused of abusing Sunni civilians in past operations.

In Ramadi, Shiite militias stayed out of the fight entirely. In Fallujah, even their participation on the sidelines could be a problem, according to Patrick Martin of the institute for the Study of War, a Washington think tank.

"It's going to be more of an issue now because of their proximity to Fallujah and the fact that they view Fallujah and its residents with great suspicion," Martin said.

While there was swift progress securing Fallujah's outskirts, pushing into the city has been much slower. It is expected to slow even further the closer that the forces get to the city center.

"When you get into the dense urban terrain, it's going to be hot and it's going to be tough work," said Col. Christopher Garver, the Baghdad-based spokesman for the U.S.-led coalition against the Islamic State group.

So far, Garver said, the speed of the operation has been determined by how much coalition air support Iraqi troops are getting. As they move into urban areas, calling in airstrikes will become more difficult due to the presence of trapped civilians and the threat of inadvertently hitting friendly forces.

Martin said a drawn out battle will not only have humanitarian consequences, it could also allow IS to regroup and launch even more debilitating counterattacks.

"The longer the operation goes on for, the more likely ISIS will be able to regenerate attack capabilities," he said.

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Associated Press writer Susannah George in Baghdad contributed to this report.