SpaceX received orders Friday from the US space agency to send astronauts to the International Space Station in the coming years, helping restore US access to space, NASA said.

The announcement was a formal step in a process that began earlier this year when Boeing was given the nod by NASA to send crew to the orbiting outpost by late 2017.

Both Boeing and SpaceX have received billions in seed money from NASA to restore American access to the ISS, after the US space shuttle program was retired in 2011.

The announcement of $4.2 billion for Boeing and $2.6 billion for SpaceX was made in September 2014.

Boeing announced in January that it would be the first, sending a piloted mission aboard its CST-100 Starliner capsule by late 2017.

However, in Friday's announcement, NASA said that "determination of which company will fly its mission to the station first will be made at a later time."

SpaceX is already flying cargo missions to the ISS, and is working on a crew version of its Dragon capsule to carry astronauts.

The announcement marked the "second in a series of four guaranteed orders NASA will make under the Commercial Crew Transportation Capability (CCtCap) contracts," the US space agency said.

"The Boeing Company of Houston received its first crew mission order in May."

Kathy Lueders, manager of NASA's Commercial Crew Program, said "it is important to have at least two healthy and robust capabilities from US companies to deliver crew and critical scientific experiments from American soil to the space station throughout its lifespan."

The ISS is expected to remain operational until 2024.

Since the shuttle program ended in 2011, the world's astronauts have relied on Russia's Soyuz capsules for transport at a pice of some $70 million per seat.

"The authority to proceed with Dragon's first operational crew mission is a significant milestone in the Commercial Crew Program and a great source of pride for the entire SpaceX team," said Gwynne Shotwell, president and chief operating office of SpaceX.

"When Crew Dragon takes NASA astronauts to the space station in 2017, they will be riding in one of the safest, most reliable spacecraft ever flown. We're honored to be developing this capability for NASA and our country."