The girl was walking across a field near Sheffield Road, Unstone, about 3pm, on Wednesday, when she was attacked from behind.

A Derbyshire Constabulary spokesman said: "The offender had walked past the girl then came back and attacked her from behind.

"He punched her in the face and grabbed her breasts and bottom and as she screamed he let go and she ran off to raise the alarm."

The offender was described as white, 5ft 4ins tall, aged in his late teens, wearing a scarf over his mouth, a hooded top and black tracksuit bottoms.

A police spokesman added: "There may have been other people in the area at the time, particularly some who were also walking a dog, and we're urging them and anyone who may have seen anything to contact police as soon as possible."

Witnesses or anyone with information should call Derbyshire Constabulary on 0345 123 3333 or Crimestoppers in confidence on 0800 555 111.