Iraqi military forces prepare for an offensive into Fallujah to retake the city from Islamic State militants in Iraq, Friday, June 3, 2016. The battle for Fallujah, now two weeks old, is shaping up unlike others in the Iraqi military's town-by-town war with the Islamic State group: Airstrikes have been sparingly employed, potentially violent Shiite militias have so far been kept to the perimeter, and the initial advance on the symbolically important town has been slow. (AP Photo/ Khalid Mohammed)
Iraqi military forces prepare for an offensive into Fallujah to retake the city from Islamic State militants in Iraq, Friday, June 3, 2016. The battle for Fallujah, now two weeks old, is shaping up unlike others in the Iraqi military's town-by-town war with the Islamic State group: Airstrikes have been sparingly employed, potentially violent Shiite militias have so far been kept to the perimeter, and the initial advance on the symbolically important town has been slow. (AP Photo/ Khalid Mohammed) Credit: Khalid Mohammed

CAMP TARIQ, Iraq — 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.