Up until the last couple of days, the US had relied on high-altitude bombing for its airstrikes in Syria, using a range of Air Force and Navy bombers (including the B-1 Lancer intercontinental bomber) in its strikes on ISIS within Syria. While carrying a lot of firepower, in the absence of forward observers on the ground, bombing from altitude runs the risk of hitting friendly forces, especially since ISIS is no longer flying its black flags.[Correction: I originally posted they were used at Kobanê. They're being used nearer to Baghdad.]
Only in the last couple of days has the US added AH-64 Apache attack helicopters to the mix. The Apache is a powerful weapon capable of precision strikes at low level, but unlike the fixed-wing aircraft, which fly far above the altitude reachable by ISIS weapons, the Apache is vulnerable to weapons ISIS has, including shoulder-launched SAMs, anti-aircraft artillery, and (though it is well-armored) potentially even small-arms fire. The odds of US fliers becoming casualties is thus greatly increased, and there are reports that ISIS is already circulating information on how to bring down the Apache.