Experts say a cluster of some 20 great white sharks was recently spotted by the US coast guard off the coast of northern California.

The San Francisco Chronicle reported on Saturday that the sharks were seen swimming together last week, about 100 yards offshore near the city of Pacifica.

A Gulf of the Farallones National Marine Sanctuary spokeswoman, Mary Jane Schramm, said two coast guard helicopters flying 500ft above the ocean spotted the group on 16 October.

Schramm said most of the sharks were between 10ft and 15ft and a couple of them were about 18ft.

The region’s sharks can grow up to 21ft and weigh as much as 7,000lbs. They typically return from the deep ocean to feed off the Farallon Islands at this time of year.