Retailer Amazon has teamed up with car companies Volvo and General Motors to deliver packages directly to users' vehicles.

With the Amazon Key app, car owners can now give delivery persons access to their cars to drop of parcels via a keyless entry and remote locking system.

The option is being offered in 37 US cities to owners of vehicles made by Volvo and General Motors (GM) brands Chevrolet, Buick, GMC and Cadillac, who have cars with internet-connected capability.

Users download the app, log in to their Amazon Prime account and pair it with their connected car service account.

After choosing their items, they then select "in-car delivery" on the checkout page, with availability to opt for same-day, two-day or standard shipping. Amazon says the service works with "tens of millions of items sold on Amazon.com".

The cars have to be parked in a publicly accessible area, like at a home or workplace, for the service to work.

Via the app, users can check to see if they are parked within the delivery range, track their package through notifications, and see what time their car was unlocked and locked again.

"Amazon uses multiple layers of verification to ensure the security of in-car deliveries," said a statement from the company.

"Each time a delivery driver requests access to a customer's vehicle, Amazon verifies that an authorised driver is at the right location with the right package, through an encrypted authentication process. Once this process is successfully completed, the car is then unlocked."

Volvo is already operating a similar service in Sweden, but with this expansion to the US, Amazon believes it will open up the opportunity to seven million drivers.

Launched in October 2017, Amazon Key also allows staff to access customers' homes and leave purchases inside.

It is one of several technologies that the Seattle-based company is rolling out, with others including grocery stores with no checkouts and deliveries by drone.