Helicopter warns California paddleboarders they're next to sharks

Swimmers at a California beach were told to get out of the water after 15 great white sharks were spotted by a circling police helicopter.

The video, filmed on board the chopper and posted by the Orange Country Sherriff's Department, shows the animals close to shore in the same area as beachgoers on Wednesday (local time).

One of the officers relays a message to those on the ground along the coastline from Capistrano Beach to San Onofre State Beach warning them to evacuate.

"Please be advised this area is heavily populated at the moment with large great white sharks, some of which are in the surf line. Please do not enter the water," the officer says.

The sharks were seen in the same area as a shark attack two weeks earlier.

There were also a number of other notices placed on beaches warning those on the ground level about the presence of sharks in the area.

In nearby Long Beach, at least four juvenile great whites were seen in shallow waters off the coast.

On the same day, US Marine base Camp Pendleton posted a photo to Facebook saying there'd been an "aggressive" shark seen at a popular surf spot on San Onofre State Beach in which someone was attacked by a shark on April 29.

It's thought warm waters and a good food supply are attracting the sharks close to shore.