A forensic expert has told a Chicago court that the sex tape at the centre of singer R. Kelly’s child pornography trial is “real,” after finding no evidence that it had been tampered with.

The R&B star is accused of videotaping himself having sex with a 13-year-old girl and is currently standing trial on 14 counts of child pornography. He has pleaded not guilty to all charges.

Kelly's defence team has previously claimed the singer could have been digitally "morphed" into the video footage using special effects.

But, during his testimony at a Chicago, Illinois court on Thursday, FBI forensic video expert George Skaluae told the jury that after examining the film, he found it to show "real people in a real environment".

Skaluae went on to tell the court that he had previously been asked to look at the tape, after it had been retrieved by the Chicago Police Department in 2002.

He explained that he'd found the tape to be "a copy, several generations away from the original".

A number of hidden "switch points" indicated the tape had been duplicated at least three times, resulting in the deteriorated quality, he said.

Skaluae added that it would have been "very difficult and time consuming" for someone to digitally alter the tape to show Kelly in the footage.

He told the court: "If it is not done well then it is going to be detectable.

"Sometimes we look at effects in movies and laugh and say, 'That doesn't look too good'. Those may have taken months or years."

Kelly has pleaded not guilty to 14 counts of child pornography and maintains the man in the tape is not him. The alleged victim, now 23, has also denied she is the girl in the footage.

The trial continues.