PERTH - AN AIR and sea search was launched for a swimmer believed to have been seized by a shark in the sea off an Australian beach on Saturday, police and reports said.

The 51-year-old man was snorkelling with his 24-year-old son at a popular Port Kennedy beach south of the West Australian capital Perth early in the morning when he disappeared.

Witnesses reported seeing 'something pretty violent' happening and blood in the water, said police spokesman Mark Valentine.

'Something very traumatic has happened there and we are treating it as a probable shark attack,' he said.

Police and emergency service boats and helicopters were searching the water and a large shark had been spotted in the area, the Australian Broadcasting Corporation reported.

Hopes of finding the man alive were fading as time passed, said police spokesman Greg Lambert.

'These are unfavourable circumstances. It's been over three hours now and he has not surfaced.'

The missing man was a local banker who often swam at the beach, the broadcaster said, adding that his family had requested that the shark should not be hunted and destroyed.

Police have asked the public to stay away from the beach and warned holidaymakers in Perth to exercise caution when swimming in the ocean.

A teenaged surfer was killed in a shark attack in Australia in April, taking the death toll from such incidents there since 2000 to 12, according to the US-based International Shark Attack File. -- AFP