NASA has hired U.S. firms Boeing and SpaceX to ferry more astronauts to the International Space Station (ISS), with the first crewed test flight set for as early as May 2018.

In 2014, both private firms entered in to a contract with NASA to carry out a space taxi service but were only offered two missions each.

Now Boeing and SpaceX will get six missions each, but only if their technology can pass NASA certification. At present, NASA astronauts go to and from the space station on a Russian Soyuz spacecraft and have done since 2011when NASA last flew its space shuttle mission.

"Awarding these missions now will provide greater stability for the future space station crew rotation schedule, as well as reduce schedule and financial uncertainty for our providers," said Phil McAlister, director, NASA's Commercial Spaceflight Development Division in a press release Tuesday.

"The ability to turn on missions as needed to meet the needs of the space station program is an important aspect of the Commercial Crew Program," he said.

NASA said Boeing's un-crewed flight test, known as an Orbital Flight Test, is currently scheduled for June 2018 and its crewed flight test set for August 2018.

SpaceX progress is reportedly quicker with an unmanned flight test scheduled for November 2017, followed by a crew flight test penciled in for May 2018.

Both companies are looking to launch their services from Florida's "Space Coast": Boeing from Cape Canaveral Air Force Station and SpaceX at NASA's Kennedy Space Center.