Boy, 13, attacked by shark in California while diving for lobsters

A 13-year-old boy diving for lobsters has been attacked by a shark on the southern California coast.

The boy was quickly pulled from the water by others and flown to a trauma centre after Saturday's attack, authorities said.

He suffered traumatic upper torso injuries, but the attack was "nonfatal," city lifeguard Larry Giles told reporters at Beacon's Beach in Encinitas, north of San Diego.

The attack occurred just before 7am during the opening hour of California's spiny lobster season.

The boy was attacked in about 2.7 metres of water approximately 137m offshore. He was brought to shore by three bystanders, Giles said.

"He was conscious, he was above water, and the bystanders were able to get him up on the beach ... and he was talking all the way to when he was being transported," the lifeguard said.

Witnesses estimated the shark was about 3.3m long, but the type was not known.