Urgent warning for US beaches after four unsuspecting swimmers attacked by sharks in just two days

LONG Island officials tell swimmers to beware of the beach waters around Long Island after a fourth person is bitten in two days and several shark sitings are reported.

With New York beaches packed for the holiday weekend, officials have had to close several beaches for short periods of time due to shark scares, and later, severe thunderstorms.

A school of at least 50 sand sharks was spotted just 200yards from the beach of Robert Moses State Park on Tuesday, the New York Post reported.

"The lifeguards jumped off their stands and were screaming, 'Get out! Get out! Get out!' It was crazy, people didn't know what to do," beachgoer Allison Ellers told NBC New York.

The sharks were spotted by drone in the morning, after the two shark attacks on Monday but before reports surfaced of the third and fourth attacks later that day.

On Tuesday afternoon, officials said that a 47-year-old man was swimming in chest-deep water near Quogue Village Beach when something bit his right knee.

The man did not see a shark, but police said the bite came from a "large marine animal."

He was taken to a nearby hospital for treatment and did not sustain life-threatening injuries, officials said.

A second man was bitten on the same afternoon further down the beach near Fire Island Pines.

The 49-year-old man was bitten in his right hand, police said, and was also taken for treatment to a nearby hospital.

On Monday, a 15-year-old girl was bitten on her left leg by an unconfirmed marine animal at Robert Moses Beach around 2 p.m.

She was treated for small wounds on her foot, and officials used drones to search for sharks but found nothing.

A few hours later, at nearby Kismet Beach, a 15-year-old boy was bitten on his left foot while surfing by what he described as a shark.

Officials said the boy's heel and toes were intact and he was healing at a nearby hospital.

"It's scary, this never happened when I was a kid," Ellers said about the shark sitings.

"I think it's crazy that there are so many sharks in the water," a young beachgoer told NBC reporters, with his mother adding that he had already declared that he would not be going back in the water even once officials reopened it.

Long Island officials also had to close some beaches temporarily on Tuesday because of rapidly moving thunderstorms affecting the area.

They have asked swimmers to pay attention to their surroundings and take safety warnings seriously when at the beach this summer.