A spacecraft has been launched on a mission to bring back two astronauts who have been at the International Space Station as a Boeing Starliner returned to Earth earlier this month without them over safety concerns.

The US private firm SpaceX's Crew Dragon spacecraft blasted off from the US state of Florida on Saturday.

The SpaceX mission carried two crew members, instead of the originally planned four. That is to leave room for the Starliner crew to join them on their return.

The Starliner lifted off in June with two astronauts onboard for a test flight to the ISS. But glitches found in its thrusters forced them to remain at the space station.

The Crew Dragon members will join the Starliner crew in conducting scientific research and other activities, after they arrive at the ISS.

All four astronauts are slated to fly back to Earth next February.

The analysis of the glitches in the Starliner is ongoing, but there is little prospect of the spacecraft being put into practical use as a means of astronauts traveling between the ISS and Earth.