Sunita Williams NASA's commercial crew astronaut

Indian American Sunita Williams is among four astronauts who have been selected by NASA for commercial flights to the International Space Station (ISS) from US soil.

They will work closely with company-led teams to understand their designs and operations as they finalise their Boeing CST-100 and SpaceX Crew Dragon spacecraft and operational strategies.

Williams, Robert Behnken, Eric Boe and Douglas Hurley will be trained for commercial spaceflights that will return American launches to US soil and further open up low-Earth orbit transportation to the private sector, the US space agency said.

"These distinguished, veteran astronauts are blazing a new trail -- that will one day land them in the history books and Americans on the surface of Mars," said NASA administrator Charles Bolden.

Williams, a US Navy captain, received her commission in the Navy in May 1987 and became a helicopter pilot, logging more than 3,000 flight hours in more than 30 different aircraft.

NASA chose Williams for the astronaut programme in 1998. She spent a total of 322 days in space and currently holds the record for total cumulative spacewalk time by a female astronaut (50 hours and 40 minutes).

She now ranks sixth on the all-time US endurance list and second all-time for a female astronaut.

"Their selection allows NASA to move forward with the training necessary to deliver on President Barack Obama's ambitious plan for returning the launch of the US astronauts to US soil," said John Holdren, assistant to the President for science and technology.

"This is a new and exciting era in the history of US human spaceflight," said Brian Kelly, director of flight operations at NASA's Johnson Space Center in Houston.

The Commercial Crew Transportation Capability (CCtCap) contracts with Boeing and SpaceX each require at least one crewed flight test with at least one NASA astronaut on board.

To meet this requirement, the companies must also provide the necessary training for the crew to operate their respective vehicles. NASA is extensively involved with the companies and reviews their training plans.

"Congratulations to Bob, Eric, Doug and Sunita and welcome to the Commercial Crew team," noted John Elbon, Boeing vice president.

"We look forward to working with such a highly-skilled and experienced group of NASA astronauts as we carve a path forward to launch in 2017."

The selections are the latest major milestone in the Obama administration's plan to partner with the US industry to transport astronauts to space, create good-paying American jobs and end the nation's sole reliance on Russia for space travel.