A US law enforcement officer has been sentenced to probation without supervision after he was caught smelling a child's underwear on video.

Deputy US marshal David Timothy Moon, 50, was looking at a house for sale in Phoenix last May, ABC15 reports.

But while his real-estate agent was occupied elsewhere, he snuck into a three-year-old's bedroom, where he was caught on a nanny cam diving into the young girl's washing basket.

The footage shows him picking up the young girl's underwear, bringing it to his face and smelling them.

And once wasn't enough - he left and came back shortly after to repeat the disturbing act.

But this time he notices the camera, says "goddamit" and hurriedly leaves the room.

The girl's father got a notification of movement in the child's bedroom, saw the recording and rang the police, ABC15 reports.

Once the allegations surfaced, Moon was fired from his job. And the case started to reveal more shocking details about Moon. His girlfriend told police he threw out his computer when he learned investigators were onto him.

An email he sent to himself of "girls doing gymnastics" was also discovered.

Moon was charged on two counts of felony trespassing with sexual motivation. But he later cut a deal with prosecutors and pleaded guilty to misdemeanour trespassing, according to ABC15.

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Judge Cooper took into account his lack of criminal records, his time in the military services and work as a deputy marshal when agreeing to him serving his probation unsupervised.

But the father, who can't be named due to the nature of the case, argued for supervision.

"What if he comes back?" the dad said.

"What if he tries to hurt us or our daughter? What if he loses his job and has nothing else to lose?"

The deputy attorney officer also disagreed with his sentence.

"The actions by the defendant, in this case, show a need for evaluation, show a need for potential treatment, and that's going to go unnoticed, untreated if there's no evaluation done in this case," Deputy County Attorney Marcus Beecher said during the trial.

However, Judge Cooper denied the request.

"The court believes that he is sincere in his remorse," she said.

A forensic psychologist told ABC15 news that this behaviour is "a compulsion". He was not involved personally in this case but has assessed hundreds of sex offenders before.

"This isn't someone who's unsure and needs to look around and tries to figure out - is this thing what arouses me?" Dr Delatorre said.

"He knows immediately."

After Moon completes his probation, he can apply for his trespassing charge to be reconsidered.