Chevron said the blast occurred at about 2:30 p.m. PST (5:30 p.m. EST) (2200 GMT) at a hydrocracking unit in Richmond, about 10 miles (16 km) northeast of San Francisco.

Refinery officials said there were no serious injuries in the incident, although one firefighter was treated for back strain and another for heat exhaustion suffered while battling the blaze.

"Everyone is accounted for," Jim Braden, a plant spokesman, told Reuters. "This thing is slowly coming under control."Chevron's Richmond refinery employs more than 1,500 people and is one of the largest refineries on the US west coast, with a capacity of 240,000 barrels per day.

Officials said the explosion appeared to have occurred in the refinery's "isomax" unit, which processes heavy fuels into gasoline and jet fuel.Local residents were told to stay safely inside theirhomes and offices as firefighters struggled to contain acrid smoke and fierce flames from the second major accident at a northern California oil refinery in as many months.

Last month, four workers were killed after a fireball erupted at another refinery in Contra Costa County near San Francisco operated by Tosco Corp.Chevron said in a statement it was investigating the cause of the explosionand did not expect the blast to have a "significant effect" on its ability to supply petroleum products to its customers.

After about three hours of intense firefighting efforts, company officials said the blaze appeared to be coming under control.News reports from the scene said at least nine people complained of breathing difficulties after they inhaled smoke, thick columns of which could be seen miles away by office workers sitting in skyscrapers in downtown San Francisco.

Firefighters went door-to-door in neighborhoods around the refinery, warning people to stay in their homes and offices and to avoid breathingthe smoke."We tell people to take shelter in place, go inside, close all doors and windows, and turn off any air conditioning," Lou Pascalli of the Contra Costa County Health Department told Reuters.

He said that while the thick, dark smoke was acrid and could cause breathing difficulties, it did not appear to be particularly toxic.West Coast oil traders reported gasoline prices were "going through the roof" on word of the blast.