The video shows an officer hitting the 20-year-old twice

Footage has emerged of a police officer punching a woman in the head on a Jersey Shore beach in the US.

The video shows the officer hitting the 20-year-old twice after pinning her down on the sand on Saturday.

People can be heard shouting "stop resisting" at the beach in Wildwood, New Jersey.

One officer can been seen kneeling by the woman as they get her on her back while another pulls her legs straight.

A number of officers have been reassigned to administrative duty while the incident is investigated.

Wildwood Police said the suspect, Emily Weinman, of Philadelphia, has been charged with underage drinking, two counts of aggravated assault on a police officer, disorderly conduct and resisting arrest.

She is also charged with spitting at an officer.

This unedited video contains some bad language and scenes some readers may find distressing:

 Police chief Robert Regalbuto said he found the footage of the arrest "alarming" but did not want to "rush to any judgement" until the investigation is completed.

Wildwood mayor Ernie Troiano Jr described the incident as a "shame".

He added that police would soon release body camera footage, which he said would reveal officers were insulted and spat on.