WASHINGTON (AP) — Difficult to resolve safety and security obstacles may prevent the Federal Aviation Administration from meeting a deadline to allow civilian drones routine access to U.S. skies within three years, according to a report released Tuesday by a government watchdog.

The FAA is under pressure from Congress, industry and other government agencies to open domestic airspace to unmanned aircraft so that they can perform a seemingly endless list of tasks that are too expensive or too risky to use aircraft with human pilots. The biggest market is expected to be state and local police departments. Others interested in using drones are farmers who want help monitoring their thirsty crops, oil companies wanting to keep an eye on pipelines and even real estate agents needing to monitor their properties.