SpaceX has successfully attached its unmanned Dragon cargo ship to the International Space Station.

Dragon is the first commercial vessel to visit the space station and is also the first ship from the United States to go to the orbiting laboratory since NASA retired its shuttles last year.

The privately-funded Dragon capsule arrived at the ISS for an historic docking last night and was captured by astronauts wielding a giant robot arm, making the first commercial delivery into the cosmos.

The capsule is delivering about a half tonne of supplies and science experiments for the ISS, and aims to return a slightly larger load of gear to Earth on 31 May.

"There was so much that could have gone wrong, and it went right," SpaceX chief executive Elon Musk, a billionaire internet entrepreneur, told reporters after the berthing was complete.

"This is really going to be recognised as a significantly historical step forward in space travel, and hopefully the first of many to come."

The Dragon is carrying 521kg of supplies for the space lab, including food, computers, utilities and science experiments. It plans to return a 660kg load to Earth.

The hatches opened earlier today so that the unloading could begin.