A South African prosecutor accused Oscar Pistorius on Thursday of lying and altering his story when the Olympic and Paralympic athlete described the night he shot dead his girlfriend Reeva Steenkamp on Valentine's Day last year.

The double-amputee sprinter faces life in prison if convicted in the Pretoria High Court of the murder of Steenkamp.

Pistorius says he shot Steenkamp in a tragic accident, firing at what he thought was an intruder hiding behind a toilet door in his luxury Pretoria home on February 14 last year.

Prosecutor Gerrie Nel cross-examined Pistorius while looking through photographs of the couple's bedroom taken after the shooting.

Nel pointed out a series of objects in one picture which ran counter to Pistorius' account of events. In response, the sprinter accused the police of tampering with the scene but Nel ridiculed this suggestion.

"Your version is so improbable that nobody would ever think it's reasonably possibly true ... Your version is a lie," Nel said.

Reuters