Two swimmers were apparently attacked by sharks off the shores of Long Island on Tuesday, a day after two others reported being attacked while enjoying the water at popular New York beaches.

At least one beach delayed opening to holiday revellers on Tuesday, after officials said drones spotted some 50 sand sharks that morning near a popular beach park.

When the beach reopened, swimmers were advised to stay close to shore.

Long Island State Parks regional director George Gorman told Newsday: “We want to make sure swimmers are safe.”

The beach was closed once more after a possible shark sighting, but officials determined it was a dolphin.

After a several attacks last year, state parks officials have increased patrols and deployed more drones to scout the waters for possible danger.

“We did have a season last year where we had six swimmers bitten from sharks, so this has turned into a bit of a concern,” Suffolk County Police Commissioner Rodney Harrison told WABC.

Tuesday’s incidents happened about 60 miles apart, including one off Fire Island Pines – not far from another attack the day before when a 15-year-old reported being bitten on the foot by a shark while surfing.

Earlier on Monday, another 15-year-old girl was treated for an apparent shark bite to a leg.

“She didn’t see what bit her, the lifeguards didn’t know what bit her, the drone operator checked the area, we didn’t see,” Mr Gorman said. “So we can’t definitively say what bit her.”

Tuesday’s shark encounters both occurred just before 2pm local time (7pm BST).

A 47-year-old man was in chest-deep water off Quogue Village Beach in the Hamptons when he felt an apparent bite to his right knee, Quogue police said.

He told authorities that he did not see a shark.

As a precaution, Quogue police advised swimmers to stay out of the water until officials could assess the risks.

Soon after, several miles away, a 49-year-old man reported having a hand bitten while swimming near Fire Island Pines Beach.

Despite the attacks, holiday revellers remained on the beach.

“That’s nature and maybe we are taking over their domain and they don’t like it,” 90-year-old Diana Fratello told WCBS.