A 20-year-old body boarder was killed Thursday when a giant shark estimated at up to five metres (16 feet) long bit off his leg in a Cape Town bay, in South Africa's second deadly mauling this year.

David Lillienfeld's brother Gustav tried to pull him to safety when the shark struck while the siblings were bodyboarding, reported Eyewitness News website.

"The deceased man sustained complete amputation of his right leg, below the hip, while body boarding with his brother and friends," said National Sea Rescue Institute spokesman Craig Lambinon.

"There are no other bite marks or lacerations on the deceased man's body -- only the complete amputation of the right leg and the leg has not been recovered."

Witnesses reported a single shark, thought to be a Great White and measuring four to five metres, attacked the man. They said up to six sharks were spotted after the incident at a popular surfing and body boarding site in False Bay.

The South African man's body was recovered off rocks on the shore after he was hauled to shore, said Lambinon.

City officials confirmed the fatal attack at Kogel Bay Resort which is near Gordon's Bay, about 50 kilometres (30 miles) from the city centre, in the eastern stretch of the large bay.

The attack is the latest shark blow for the top tourist city after a British man lost parts of both legs at Fish Hoek in September, after he ignored warning flags on a beach which had been closed due to shark activity.

Officials are mulling a trial shark net for Fish Hoek, a popular swimming beach where three attacks, two deadly, have taken place.

To counter attacks due to regular shark presence, the city also uses a unique shark spotting programme which monitors for sightings of the apex predators.

Last week, coastal authorities warned beachgoers around the tourist town of Knysna that sharks were moving into the area, feeding on the remains of a whale that had run ashore about 400 kilometres (250 miles) east of Cape Town.

A shark research permit for a National Geographic documentary being filmed in the area was cancelled after the attack.

The attack is the second shark fatality along South African shores this year after a swimmer was killed in waist-deep water at Second Beach in the rural Eastern Cape in January.