The victim was spotted by a lifeguard as he was floating encircled by sharks off Stuart Beach, some 90 miles north of Miami, the local news website TCPalm.com said. The local man had been kite surfing on an unguarded stretch of beach when the attack occurred.

The lifeguard paddled out to him and brought him to shore on his board but the man, identified as Stephen Howard Schafer, appeared to have multiple shark bite wounds and was unconscious when rescued. Paramedics tried resuscitate him on the beach before he was rushed to hospital, where he subsequently died.

Mr Schafer, 38, was kite surfing south of Stuart Beach about 4:15 pm when the sharks attacked him, said Bureau Chief Doug Killane of Martin County Fire-Rescue.

The Martin County Sheriff's Office is investigating the death. Capt. Mark McKinley said shark attacks in the region were very rare.

"I've been here 25 years," Capt McKinley said. "To my knowledge, this is the first shark-related fatality we've seen."

Thousand sharks have been spotted off the Florida coast in recent days, leading to the closure of beaches.

The increase in shark sightings is due to their migration pattern.