President Barack Obama's 2013 budget request for Nasa would slash spending on Mars exploration and shift funds to human spaceflight and space technology.

As reported by BBC News last week, this means the US will pull the plug on its joint missions to Mars with Europe.

If approved by Congress, the budget request would reduce funds available for planetary science by about 21%.

But spending on human exploration and space technology would rise by 6% and 22% respectively.

"There's no doubt that tough decisions had to be made," Nasa's administrator Charles Bolden told a news conference in Washington DC.