The United States has stationed about a half-dozen Apaches at Baghdad International Airport, but they have been used only rarely in the two-and-a-half-month-old aerial campaign against the Islamic State, in part because of worries about their vulnerability to ground fire and because of a lack of American search-and-rescue teams in Iraq that could respond to downed aircrews. The concerns also reflect the White House’s insistence on limiting the number of American troops in Iraq and their exposure to hostile fire.

This month, Apaches entered the battle for the first time, in coordination with United States Air Force jets, to carry out four airstrikes on a large Islamic State force northeast of Falluja, in the sprawling desert and agricultural province of Anbar. The militants have established several strongholds there, and have continued to gain ground there against Iraq’s security forces in recent weeks.

Now, though, the Iraqi military is beginning to mount larger and more complex efforts around the country to retake territory from the Islamic State, including a counteroffensive that began a week and a half ago to break the militants’ stranglehold on a key refinery in Baiji, north of Baghdad. The new phase will mean an increase in the frequency of combat missions by coalition aircraft, and will likely demand a greater use of lower-flying American attack helicopters and gunships, which have important advantages in urban warfare.

Since much of the most difficult fighting in the coming months is expected to unfold in the towns and cities of Anbar, American generals may be inclined to order more Apaches to support Iraqi ground troops. They may also make greater use of AC-130 gunships, a lumbering, propeller-driven plane bristling with cannons that circles at altitudes at the outer limits of some shoulder-fired missiles.

As Iraqi and American officials weigh the added risk to their aircrews and, potentially, to civilian aircraft, they are particularly concerned about the threat of shoulder-fired heat-seeking missiles, commonly known as Manpads, short for Man-Portable Air Defense Systems.

Syrian rebels have amassed multiple Manpad models since 2012, and the Islamic State has generally had little trouble acquiring any weapon used by Syrian rebels either through purchase or capture, military analysts say. Though the Pentagon’s Central Command acknowledges this concern, it said it had no conclusive evidence yet that the Islamic State had such weapons.