"The hatches between the International Space Station's Harmony node and the Dragon spacecraft were opened at 09:53am GMT, Nasa said in a statement.

"The Expedition 31 crew members will now begin several days of unloading and reloading operations with the world's first commercial resupply vehicle."

SpaceX on Friday became the first commercial outfit to send its own unmanned cargo capsule to the International Space Station, heralding the start of a new era for private spaceflight.

With no humans on board, the Dragon capsule is delivering about a half ton of supplies and science experiments for the ISS, and aims to return a slightly larger load of gear to Earth on May 31.

The US space shuttle program ended in 2011, leaving only Russia capable of carrying astronauts and cargo to the ISS and back to Earth.

The space agencies of Japan and Europe have supply ships that can ferry cargo to the ISS but cannot return to Earth intact, and those missions are set to end in the coming years.

That means SpaceX and its competitor Orbital Sciences Corporation would likely become the chief cargo servicers of the $100 billion space station, which is set to remain operational until 2020, Nasa said.