Nasa has extended its contract with Russian federal space agency Roscosmos to carry American astronauts to the International Space Station (ISS).

The deal is worth $490m and allows for the transportation of US astronauts to the ISS, amid a lack of Congressional funding.

Modifications to the contract, which is valid until 2019, highlight the US’s reliance on Russia exclusively.

Since July 2012, the US has been dependent on Russia for carrying astronauts to the ISS, as the country stopped its own space shuttle programme.

The decision to pay and use Russian space vessels was made due to the US federal agency falling short of funding.

" The fastest path to…ending our sole reliance on Russia is fully funding Nasa’s Commercial Crew Program in FY 2016."

Going forward, the agency will not be able to acquire private vessels from firms such as Boeing and SpaceX for transportation, as has been the case for the last five years.

According to a letter to Congress from Nasa Administrator Charles Bolden, Boeing and SpaceX may be unwilling to develop plans to launch their first manned vessels by the end of 2017, if the 2016 budget denies the funding requested by President Barack Obama.

Costs for the programme are estimated at $1.24bn.

Bolden said: "The fastest path to bringing these new systems online, launching from America, and ending our sole reliance on Russia is fully funding Nasa’s Commercial Crew Program in FY 2016."

Republicans in the House of Representatives and Senate have proposed reductions to the amount by $250m and $300m respectively, reports Agence France-Presse.

Bolden added that Congressional decision to cut funding in accordance with the Republican proposals is likely to: "result in funds running out for both contractors during the spring / summer of FY 2016."