Amazon has been granted permission to test its latest delivery drones over U.S. airspace, according to Digital Spy.

The Federal Aviation Administration has granted the retailer's request to fly the unmanned aircraft domestically, but the agency has imposed restrictions on the trials.

The drones cannot be flown higher than 400 feet during test flights and must not travel at more than 100 miles per hour.

Amazon previously received FAA permission for drone tests back in March, but by the time the request was approved, the prototype aircraft it was hoping to trial had become obsolete, reports Reuters.

The firm has already tested its updated drone technology in Canada, and now has clearance to move its field research to its native USA.

Amazon's delivery drones are designed to carry packages weighing up to 20kg to customers within 30 minutes of an order being placed.