Singer Keith Urban has said he is "horrified" by reports of a rape during his US concert.

The comment by Keith, who is married to actress Nicole Kidman, was made in a Facebook posting.

"Our hearts and prayers go out to all those affected. This type of behaviour stands in stark contrast to the spirit of our shows," he added.

Sean Murphy, 18, of Boston has pleaded not guilty to a rape charge filed after concert-goers told police they saw what they believed to be a sex attack on the lawn at the Xfinity Centre in Mansfield, Massachusetts. Bystanders captured the scene on mobile phone cameras.

Murphy says the incident was consensual, but his alleged victim told a bystander who intervened that she had not consented.

Several dozen people were treated for alcohol-related illnesses at the July 26 concert south of Boston.