WASHINGTON (Reuters) - The number of unmanned aircraft, or drones, in the United States will jump dramatically over the next five years, the U.S. Federal Aviation Administration said on Tuesday.

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The increase comes after the Obama administration in 2016 implemented new rules that opened the skies to low-level small drones for education, research and routine commercial use. Policy makers are still debating whether to allow a sweeping expansion in drone use for activities like deliveries where aircraft would fly beyond the sight of an operator.

The FAA said it estimates the fleet of small hobbyist drones will more than triple from an estimated 1.1 million vehicles in 2016 to more than 3.5 million by 2021. The agency also estimates the commercial drone fleet will grow from 42,000 at the end of 2016 to about 442,000 aircraft by 2021. The aviation safety agency said there could be as many as 1.6 million commercial drones in use by 2021.

The FAA said Tuesday the key difference in its estimates of commercial drone growth is in “how quickly the regulatory environment will evolve, enabling more widespread routine uses of (drones) for commercial purposes.”

The FAA on Tuesday also predicted the number of pilots of drones is expected to increase from 20,000 in 2016 to a range of 10 to 20 times as many by 2021.

Since August, the FAA has approved more than 300 waivers for drone use without some restrictions, including Union Pacific Railroad, BNSF Railway Co owned by Berkshire Hathaway, Intel Corp, Walt Disney Parks and Resorts, Time Warner’s HBO and CNN units.

Transportation Secretary Elaine Chao told firefighters in a speech this month that “while drones have a lot of potential to assist responders, they can also pose a problem if not carefully monitored.”

Current drone regulations require a certified pilot to stand ready to intervene in any commercial drone flight and keep a line-of-sight view of the aircraft.

Both Amazon.com Inc and Alphabet Inc’s Google unit have been exploring the use drones to deliver goods ordered online.

The White House said last year unmanned aircraft could lead to $82 billion in economic growth by 2025 and support up to 100,000 jobs.

The August rules were aimed at allowing drone use for agriculture, research and development, educational and academic use, and powerline, pipeline and antenna inspections, along with aiding rescue operations, bridge inspections, aerial photography and wildlife nesting area evaluations.