A swimmer has been killed in a shark attack at a beach in Perth, on the west coast of Australia, police have said.

Emergency crews were called to a "shark-related incident" at Sorrento Beach at around 10.15 local time on Friday.

Police said an extensive search of the area was carried out by authorities in helicopters and on boats but did not say whether a body had been found.

One witness described seeing a man being pulled under the water by a shark, just 50 metres off the shore, while another said they saw a "pool of blood" in the water, according to local media.

One witness told local outlet WAtoday that she saw the man being "mauled" as the shark "tossed him around like a fish".

"I'm a nurse, so I mean I obviously see a lot of gruesome stuff, but to see somebody being mauled to death in the water in front of me is pretty hard to clock," she told WAtoday.

"I only live just down the road, and I swim every day. I mean, there's no way I'm going to be getting in that water again."

Another witness said she was walking along the beach with a friend when the attack happened.

"I looked out to the ocean and I could see a pool of blood," Jacqui Rapaic told national broadcaster ABC.

She said she believed two people had been swimming, but only one had made it back to shore.

Members of the public had reported seeing a 2.5-metre-long shark at around the same time.

The beach remains closed to the public and authorities have asked the public to avoid the area.

There have been multiple fatal shark attacks in Australia over the past year, including on a 12-year-old boy in Sydney in January.