Four people were killed and at least 20 were injured in a shooting at a restaurant in a South Carolina island town on Sunday, the Beaufort County Sheriff’s Office said.

Deputies were called to Willie’s Bar and Grill on St. Helena Island just before 1 a.m. and found several people suffering from gunshot wounds, the sheriff's office said in a statement.

Of those injured, four are in critical condition, authorities said.

The sheriff's office said it was investigating the incident but declined to provide further details. The office declined to release the names of those killed pending notification of family members.

Hundreds of people were there at the time of the shooting, the sheriff's office said.

"This is a tragic and difficult incident for everyone," the sheriff's office said.

St. Helena Island is known as an epicentre of a culture of people descended from African slaves known as Gullah Geechee. The bar and grill where the shooting occurred describes itself as serving authentic Gullah cuisine.

Mass shootings, which the Gun Violence Archive defines as incidents where four or more people are shot, have become more common in the U.S. in the past decade.

Americans are largely divided along political lines regarding possible policy solutions, with Democrats favouring more gun restrictions and Republicans backing gun rights and better enforcement of laws against violent crime.