By Deepa Seetharaman

SAN FRANCISCO, Dec 8 (Reuters) - Amazon.com Inc plans to shift more of its drone testing outside U.S. borders unless it gets quick permission from U.S. regulators to conduct outdoor trials, the company said in a letter to the Federal Aviation Administration late on Sunday.

The U.S. online retailer has already started conducting outdoor tests "in other countries with regulatory environments more supportive of small (unmanned aircraft systems) innovation," according to the letter written by Amazon vice president of global public policy Paul Misener.

Amazon says outdoor testing is crucial to developing its "Prime Air" program, which aims to use drones - small unmanned aircraft - 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, first reported by The Wall Street Journal.

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. Amazon said there were dozens of U.S. job openings for its Prime Air division for hardware engineers and research scientists.

Last year, the U.S. government created six sites for companies, universities and others to test drones for broader commercial use. The area near Seattle where Amazon wants to conduct its tests is not among those sites, and Amazon said it would be impractical for "it to pursue the test sites as our sole method of testing at this time." (Reporting by Deepa Seetharaman. Editing by Andre Grenon)