An apparent explosion in a Rhode Island beach has wounded a woman and prompted immediate evacuation, authorities say.

The explosion, which reportedly occurred around 11:15 on Saturday, prompted officials to evacuate the Salty Brine beach in the town of Narragansett.

The 50-year-old woman, who was hurt, had been sitting in a chair up against breakwater at the tide line when the explosion blew her "onto the rocks". She was taken to a hospital and her condition was not known.

An investigation is underway and officials are searching for someone they think had planted something harmful in the sand.

"We believe there had been a small explosion, sub-surface, that erupted out of the sand and virtually caused one of our beach patrons to ultimately fall backwards and forwards," said Larry Mouradjian of the state department of environmental management.

"We have no evidence or indication that there was a device, this could be natural, it could be man-made, at this point, it's still under investigation," said Kurt Blanchard, the acting chief of the Rhode Island Environmental Police.

Dave Dean, a local radio host, was on the beach at the time the explosion happened. He said "I thought it was a small tremor or something."

"I didn't think anything of it. And I started hearing people start yelling 'Call 911' because a lady got hurt in the incident. I saw a lady on the ground, and after about 20 minutes they asked us to get off the beach."

So far, no other information has been provided by the team of investigators, which include police, bomb squad, and the Bureau of Alcohol, Tobacco, Firearms and Explosives.