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.

Southern California Edison, which operates the plant just south of San Clemente, said the leak was detected at about 3 p.m. in a water purification system for one of the two nuclear reactors.

The utility said an alert was issued because fumes could prevent access to certain areas of the plant, but that there were no injuries. Employees were evacuated as a precaution.

The alert was declared over shortly after 6 p.m., SCE said, and about 25 gallons (95 liters) of ammonia were collected in a basin beneath the tank.

No radioactive material was released, SCE said.

Nuclear Regulatory Commission spokesman Victor Dricks said the ammonia was leaking at the rate of about one gallon every ten minutes and can turn into gas on contact with air, prompting 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.

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.

The San Onofre plant consists of two pressurized water reactors.