SpaceX, the private rocket company of billionaire entrepreneur Elon Musk, launched two Americans toward orbit from Florida on Saturday in a mission that marks the first spaceflight of Nasa astronauts from US soil in nine years.

A SpaceX Falcon 9 rocket lifted off from the Kennedy Space Center at 7.22pm GMT, launching Doug Hurley and Bob Behnken on a 19-hour ride aboard the company's newly designed Crew Dragon capsule bound for the International Space Station.

Moments before liftoff, Hurley said, 'SpaceX we're go for launch. Let's light this candle', paraphrasing the famous phrase uttered on the launch pad in 1961 by Alan Shephard, the first American launched into space.

Crew Dragon separated from its second stage booster at 3:35 and minutes later entered orbit.

The craft launched from the same pad used by Nasa's final space shuttle flight, piloted by Hurley, in 2011.

'It's incredible, the power, the technology', said US President Donald Trump, who was at Kennedy Space Center at Cape Canaveral in Florida for the launch, 'That was a beautiful sight to see'.

The mission's first launch try on Wednesday was called off with less than 17 minutes remaining on the countdown clock. Weather again threatened Saturday's launch, but cleared in time to begin the mission.

Nasa chief Jim Bridenstine has said resuming launches of American astronauts on American-made rockets from US soil is the space agency's top priority.

”I'm breathing a sigh of relief, but I will also tell you I'm not gonna celebrate until Bob and Doug are home safely.” Bridenstine said.

For Musk, the launch represents another milestone for the reusable rockets his company pioneered to make spaceflight less costly and more frequent. And it marks the first time commercially developed space vehicles - owned and operated by a private entity rather than Nasa - have carried Americans into orbit.

The last time Nasa launched astronauts into space aboard a brand new vehicle was 40 years ago at the start of the space shuttle program.

Hurley, 53, and Behnken, 49, Nasa employees under contract to fly with SpaceX, are expected to remain at the space station for several weeks, assisting a short-handed crew aboard the orbital laboratory.

Boeing Co, producing its own launch system in competition with SpaceX, is expected to fly its CST-100 Starliner vehicle with astronauts aboard for the first time next year. Nasa has awarded nearly $8 billion to SpaceX and Boeing combined for development of their rival rockets.

Trump called the launch the beginning, saying that eventually there would be flights to Mars.

He was joined at the viewing by Vice President Mike Pence, Commerce Secretary Wilbur Ross, Education Secretary Betsy DeVos, Florida congressman Matt Gaetz and Senator Rick Scott.

During the drive, Behnken and Hurley passed former astronaut Garrett Reisman holding a side saying,“Take me with you.”