Amazon has said it will shift more of its drone testing outside U.S. borders unless it gets quick permission from US regulators to conduct outdoor trials.

In a letter to the Federal Aviation Administration, Amazon said that other countries with regulatory environments were more supportive of small unmanned aircraft system innovation. Amazon vice president of global public policy Paul Misener said that outdoor testing is crucial to developing its "Prime Air" program, which aims to use drones to deliver packages in 30 minutes or less. It said it preferred to keep that testing within the United States.

In July, Amazon sought permission from the FAA to test drones in outdoor areas near Seattle, where one of its research and development labs is working on the technology, but the FAA has been slow to give its approval.

"Without approval of our testing in the United States, we will be forced to continue expanding our Prime Air R&D footprint abroad," Misener wrote in the letter.

Drones are among several initiatives underway at Amazon to help control rising shipping costs and compete with brick-and-mortar stores by delivering items quickly.