ASUNCION (AFP) - An influential politician's sex tape with three young women has caused a scandal in Paraguay, where prosecutors have opened an investigation into whether one of the participants was a minor.

The cellphone video, widely shared on social networks, stars veteran senator Juan Carlos Galaverna, 64, who is known for a straight-talking style and is a key player in President Horacio Cartes' conservative party.

One of the women in the tape has told the media she was only 16 when it was made.

"We're going to check if the girl was a minor," prosecutor Teresa Martinez said on Tuesday. "If it's established that he had sexual relations with a minor, he could be prosecuted."

She said the crime was punishable by a fine but did not carry a prison sentence.

The young woman at the centre of the scandal has told journalists the video became public when her cellphone was stolen.

Galaverna downplayed the episode on Tuesday.

"Let him who is without sin cast the first stone," he told broadcaster Radio 1000.