Workers were evacuated from a unit at the San Onofre nuclear power plant in California on Tuesday after an ammonia leak, but the incident posed no threat to the public, officials said.

Liquid ammonia began leaking from a storage tank at about 3 p.m. local time, prompting an alert, Nuclear Regulatory Commission spokesman Victor Dricks said.

Dricks said the ammonia, which was leaking at the rate of about one gallon every ten minutes and turns into gas on contact with air, prompted the evacuation of employees at a turbine building.

Some vital areas in the turbine building became inaccessible because of the gas leak so they declared the alert, Dricks said.

No one has been evacuated from the site, nobody's been injured and nobody off the site should be affected, he said.

Dricks said the gas did not pose a danger to the public because it was a relatively small amount of ammonia that was leaking very slowly.

He said he did not know how many workers had been evacuated.

We have people at the site who are carefully monitoring the situation, Dricks said.

Representatives for Southern California Edison, which operates the seaside plant on the border of Orange and San Diego counties, could not immediately be reached for comment.

San Diego County said in a written release that it had activated and staffed its Operational Area Emergency Operations Center in order that it may be prepared should the situation at the power plant deteriorate.

The County said it was monitoring the situation but that officials were not recommending any action for the public following the leak.

There's no threat to the public and no protective action recommended, San Diego County spokeswoman Tammy Glenn said.