WASHINGTON — The Federal Aviation Administration will miss a September 2015 deadline set by Congress to allow drones to fly throughout the nation’s skies because of technical and regulatory obstacles that are unlikely to be resolved anytime soon, according to a government audit released Monday.

The FAA is significantly behind schedule in drawing up rules and standards to ensure that drones are airworthy, that pilots are trained properly and that their aircraft won’t interfere with other air traffic, the Transportation Department’s inspector general concluded in a highly critical audit.

The report said the FAA still had to confront so many hurdles that it was unclear when – or if – remotely controlled aircraft could be safely integrated into the nation’s airspace.

In 2012, Congress passed a law that effectively legalized commercial and civilian drone flights in the United States. With backing from drone manufacturers and other businesses eager to embrace a new era in aviation, lawmakers directed the FAA to come up with rules and standards to permit widespread drone traffic by Sept. 30, 2015.

The inspector general’s audit, however, highlighted the difficulties of absorbing drones into U.S. skies and concluded that “safety risks will persist” until the FAA can establish a set of rules for doing so.

A Washington Post investigation in June found that at least 49 large military drones have crashed while on training missions in the United States since 2001.