U.S.'s National Aeronautics and Space Administration (NASA) and Russian Federal Agency Roscosmos have signed a $335-million modification to the current International Space Station contract for crew transportation in 2013 and 2014, NASA said in a statement.

The contract stipulates that Russian Soyuz spacecrafts will convey U.S. astronauts to the space station after NASA scraps its Discovery shuttle launches in late September.

Space station crew members will launch on four Soyuz vehicles in 2013 and return on two vehicles in 2013 and two in 2014, NASA said.

"The firm-fixed price modification covers comprehensive Soyuz support, including all necessary training and preparation for launch, crew rescue, and landing of a long-duration mission for six individual station crew members," the statement said.

The contract, initially worth $306 million, was signed by NASA and Roscosmos in May 2009.

On Monday, the U.S. Discovery space shuttle blasted off from the Kennedy Space Center in Florida on one of NASA's final stockpiling missions to the ISS. The shuttle is expected to dock with the space station later on Wednesday.

NASA will carry out three more Discovery launches by late September and then scrap the program. The U.S. space agency says the shuttles are outdated and too expensive to maintain.

WASHINGTON, April 7 (RIA Novosti)