A US media monitoring group has issued a "nationwide parental alert" after hidden pornographic scenes were found in the popular game Grand Theft Auto: San Andreas.

The organisation, National Institute on the Media and the Family, issued the warning after it was revealed that a software modification allows players to see pornographic scenes contained in the game.

The modification is called 'Hot Coffee' and was created by Patrick Wildenborg, 36, from the Netherlands. He claims that the modification adds nothing illegal to the game and merely allows users to see what is already written into the code.

"If Rockstar Games denies that, then they're lying and I will be able to prove that," said Patrick Wildenborg, mod author, in a email to Associated Press.

"My mod does not introduce anything to the game. All the content that is shown was already present on the DVD."

Grand Theft Auto: San Andreas is one of the most controversial games of all time. In order to take part players must steal cars, deliver drugs and use prostitutes. There have been calls for it to be banned in some US states.

"It should be clear to everyone by now that video games do influence young people," said Dr David Walsh, author of the best selling book on teenage brain development, Why Do They Act That Way?.

"Grand Theft Auto: San Andreas never belonged in the hands of kids. We are taking the unusual step of alerting parents to the pornography available through this game to any child or teen who is internet savvy."