Amazon has criticised the US Federal Aviation Administration (FAA) for delaying approvals for drone testing.

The FAA has issued experimental airworthiness certificate for Amazon’s home delivery drone prototype to conduct outdoor tests.

However, the company said that it has already developed advanced models and the prototype approved had been rendered obsolete.

Speaking to Senate Subcommittee on Aviation Operations, Safety and Security, Amazon.com global public policy vice-president Paul Misener said: "We don’t test it anymore. We’ve moved on to more advanced designs that we already are testing abroad.

"We don’t test it anymore. We’ve moved on to more advanced designs that we already are testing abroad."

"Nowhere outside of the United States have we been required to wait more than one or two months to begin testing.

"This low level of government attention and slow pace are inadequate, especially compared to the regulatory efforts in other countries."

The company said it has filed a new application with the agency to approve the upgraded prototype, and intends to secure a commercial-drone exemption for more flexibility, reported Wall Street Journal.

The FAA stated that the Amazon’s second application must be analysed from a new perspective as it incorporates significantly different aircraft design.

The US had banned commercial drone flights in the national airspace with exceptions for aircraft that weigh less than 55lb.

Earlier this week, the aviation regulator has streamlined regulatory process for companies with exemptions, including a blanket approval for conducting drone flights with limited operations without the need to secure approval for each drone flight.

The FAA will grant certificate for aircraft that operate within visual line of sight of the pilots, and stay certain distances away from airports or heliports.

The proposed rules for drones, which would be finalised in early 2017, would limit drones to an altitude of 500ft.

In view of delayed regulatory approval in the US, Amazon is reportedly looking to trial its drone delivery in the UK, according to Telegraph.