The Pentagon's drone pilots are some of the best in the world. In a few short years, they've become a central part of U.S. operations overseas, conducting round-the-clock aerial surveillance and supporting U.S. soldiers at ground level. But a new federal audit also finds that the pilots are overworked and under-trained. And it's taking a toll.

The military's two most important drone fleets operate out of the Air Force and the Army. Both services suffer from a chronic shortage of pilot training, according to a Government Accountability Office report, one that makes it hard for operators of unmanned aerial systems to keep their skills sharp and even to train new drone pilots.

Army pilots are constantly being assigned to other duties that prevent them from logging the flight hours they need to stay current, the GAO said.

"Army UAS pilots in all of the focus groups we conducted stated that they had difficulty completing UAS pilot training in units because they spend a significant amount of time performing additional duties such as lawn care, janitorial services, and guard duty," the report reads.

One pilot interviewed by GAO flew an average of 12 training hours per year for the last three years — roughly half the yearly amount required by the Army.

Other constraints include a lack of equipment, a failure by higher-ups to recognize the kind of training that's required of pilots and the fact that experienced pilots are often asked to spend their time training less-experienced units.

A Defense Department spokesperson didn't immediately respond to a request for comment.

Front-line drone pilots tend to log a lot of training hours. The Air Force, for example, allows pilots to record mission time as training time. But even these pilots fly predominantly one or two types of missions out of the half-dozen they're supposed to be proficient in. As a result, the Air Force pilots are disproportionately trained in surveillance and reconnaissance, compared with, say, interdiction. Imagine if a baseball player practiced his swing all day long but never threw or caught a ball. That's what you've got in the Air Force.

The Air Force has pushed most of its drone pilots into flying operations -- to the point where there just aren't enough to go around. The service estimates that it has only 83 percent of the UAS operators it needs to work effectively, and that figure has been declining. In December 2013, the Air Force said it had 85 percent of the requisite pilots, according to the GAO.

The military has taken some steps to fix the problem. To keep pilots from quitting, the Air Force dramatically increased the monthly bonus it hands out when pilots near the end of their six-year commitments. The Army has reviewed the training problem and come out with a set of recommendations for improvement.

But some of the stopgap measures the Pentagon has implemented so far may be unsustainable, the GAO suggested. To get more training into the hands of pilots, the Army has beefed up the number of pilots learning to be drone instructors. The problem is, with so few pilots in the system with adequate experience, the Army has had to lower the entry requirements to the teachers' school.

Those prerequisites include things like a soldier's rank, a baseline number of flight hours with a drone, recent flight experience and a demonstrated level of readiness.

The Army began waiving some of these requirements in 2013 so that its instructor school could graduate more trainers. Between then and 2015, about 40 percent of students got some kind of waiver, the GAO said. That practice ended in February, but the result is that some of the military's newest drone pilots are being trained by people who themselves lack sufficient training.