Amazon is warning federal officials that it will "have no choice" but to move more of its drone research overseas if the company doesn't get approval to test its fleet of unmanned delivery vehicles in the United States — and soon.

Amazon argued in a letter made public Monday that the Federal Aviation Administration continued delay on developing drone regulations risks undermining its plans for Amazon Prime Air, not to mention the broader market for unmanned aerial systems (UAS).

"I fear the FAA may be questioning the fundamental benefits of keeping UAS technology innovation in the United States," Amazon policy executive Paul Misener wrote to the FAA. "Amazon is increasingly concerned that, unless substantial progress is quickly made in opening up the skies in the United States, the nation is at risk of losing its position as the center of innovation for the UAS technological revolution, along with the key jobs and economic benefits that come as a result."

(Amazon founder Jeffrey P. Bezos also owns The Washington Post.)

The letter comes days before a congressional hearing on the FAA's delayed effort to integrate drones into the national airspace. The agency has come under increasing pressure to accelerate its rulemaking, after it missed an August deadline to address commercial drones weighing less than 55 pounds. Lawmakers such as Sen. Charles Schumer (D-N.Y.) have called for the agency to act faster in the wake of several near-collisions between drones and traditional aircraft. Those same incidents prompted Sen. Dianne Feinstein (D-Calif.) to call for the FAA's ban on commercial drones to continue.

In the cases highlighted by Feinstein, aircraft pilots had to take emergency evasive maneuvers to avoid hitting unauthorized drones flying at thousands of feet in altitude. Amazon is asking for special permission to test its drones with a "conservative" proposal that would limit the company's activities, the company said. Amazon's drones would operate below 400 feet and within the pilot's line of sight.

Amazon has sought exemptions from the drone ban under other parts of federal law. But no other avenue appears as promising as a grant of approval the FAA began offering in September, under a portion of the law known as "Section 333." Among the first beneficiaries of that program were a number of Hollywood film companies, and Amazon wants to be given the same treatment.

But without it, Amazon said, "we will be forced to continue expanding our Prime Air R&D footprint abroad."

The FAA said Monday it was still reviewing Amazon's Section 333 application, but that it needed more company input before it could move forward on Amazon's other applications for exemption.