Last month, NASA had asked Boeing and SpaceX to suspend work in response to a protest filed with the US Government Accountability Office (GAO) by Sierra Nevada Corporation, who lost the bid to build commercial space taxis.

"The agency recognises that failure to provide the transportation service as soon as possible poses risks to the ISS crew, jeopardises continued operation of the ISS, would delay meeting critical crew size requirements and may result in the US failing to perform the commitments it made in its international agreements," NASA said in a statement.

"NASA has determined that it best serves the US to continue with the performance of the Commercial Crew Transportation Capability (CCtCap) contracts that will enable safe and reliable travel to and from the ISS on American spacecraft and end the nation's reliance on Russia for such transportation," the statement read.

While Boeing Co got a contract worth $4.2 billion, SpaceX grabbed $2.6 billion fixed-price contract, to end the US's reliance on Russian hardware by 2017.