An unmanned SpaceX rocket blasted off from Florida on 14 April to send a cargo ship to the International Space Station, then flipped around and made a hard landing on a platform in the ocean.

Video footage appeared to show the rocket exploding into a ball of fire on a customized barge stationed about 200 miles off the coast of Jacksonville, Florida.

"Rocket landed on droneship, but too hard for survival," SpaceX founder and chief executive Elon Musk posted on Twitter.

The booster's flyback, years in the making, marks another step in the company's quest to develop rockets that can be refurbished and reflown, potentially slashing launch costs.

During a previous landing attempt in January, the rocket ran out of hydraulic fluid for its steering fins, causing it to crash into the platform.

A second attempt in February was called off because of high seas, but the rocket successfully ran through its pre-programmed landing sequence and hovered vertically above the waves before splashing down and breaking apart.

SpaceX is one of two companies hired by Nasa to fly cargo to the station following the retirement of the space shuttles. It hopes to be certified to fly US military payloads by June.