JOHANNESBURG - Oscar Pistorius has appealed his murder conviction for killing girlfriend Reeva Steenkamp to South Africa's Constitutional Court, a lawyer for the double-amputee Olympian said Monday.

"We filed papers with the Constitutional Court today asking them to grant us leave to appeal," defense lawyer Andrew Fawcett said.

Pistorius' lawyers have not said on what basis they would appeal the murder conviction at the Constitutional Court, and it was not immediately clear how long the court will take to decide whether it will hear the case.

In December, an appeals court threw out a lower court's lesser manslaughter conviction and found Pistorius guilty of murder. Days later, a judge in another court set an April sentencing date. At the time, prosecution spokesman Luvuyo Mfaku said the April date would allow the Constitutional Court time to decide whether it would hear Pistorius' appeal.

The appeals court said that regardless of who was behind the door, Pistorius should have known someone could be killed if he fired multiple times. Under South African law, a person can be convicted of murder if he or she foresaw the possibility of someone dying through their actions and went ahead anyway.

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Pistorius shot Steenkamp through the door of a toilet cubicle in his home early on Valentine's Day 2013. He is currently out on bail and staying at his uncle's mansion in Pretoria.

The minimum sentence for murder in South Africa is 15 years, though a judge can reduce that sentence for what the law describes as exceptional circumstances.

A champion athlete, before the killing of Steenkamp in the early hours of Valentine's Day 2013 Pistorius had been televised at sporting events and seen in magazines in advertisements. In contrast, he has rarely been seen in public while under house arrest.