A reactor at the San Onofre nuclear power plant near San Diego has been shut down after a radiation leak which was not big enough to cause public harm, the US atomic safety agency said Wednesday.

Radioactive gas escaped from a pipe in a building located next to the reactor on Tuesday evening, the US Nuclear Regulatory Commission (NRC) said.

Since the equipment-housing building into which the gas leaked is not airtight, it is possible a "very, very low level," of radioactive material escaped into the environment, said NRC spokesman Victor Dricks.

Those traces would be "barely measurable against (existing) background levels," and would pose no danger to the public, he added.

The operator of the California nuclear plant, Southern California Edison Co. (SCE), said it was investigating the exact cause of the incident and that the reactor would not be used for several days.

At the time of the incident the other reactor at the California site was shut down for essential maintenance, but SCE said it had sufficient supplies to service its customers.

San Onofre produces enough energy to power 1.4 million homes, according to SCE.