RICHMOND, California (AP) — More than 500 workers at a Chevron Corp. refinery in the San Francisco Bay Area went on strike early Monday over safety concerns and to demand a salary increase to keep up with inflation and the area's high cost of living.

The strike affecting the refinery in the city of Richmond began at 12:01 a.m. It came after workers voted down Chevron’s most recent contract offer and the company refused to return to the bargaining table, the United Steelworkers union said.

Chevron said in a statement Sunday night that it has negotiated with the union for months and believes a contract offered by the company was fair and addressed union concerns.

The union said it had negotiated a national agreement for oil workers on wages and working conditions, but about 200 individual bargaining units still had to negotiate local issues.

USW Local 5 representative B.K. White, a refinery operator who has worked for the company for 29 years, said Chevron failed to address worker fatigue and a lack of staffing.

“If we had more people and could get a better pay rate, maybe our members wouldn’t feel obligated to come in and work as many as 70 hours a week to make ends meet. We don’t believe that is safe,” White said.

Chevron said that in Richmond, the union's demands "exceeded what the company believes to be reasonable and moved beyond what was agreed to as part of the national pattern bargaining agreement.”

The company offered a 2.5% pay increase, but the union had asked for 5% to keep up with inflation and cost of living in the Bay Area, White said.

“It's rough for the blue-collar worker in the Bay Area, and we asked for a 5% bump to help us out a little bit with our medical at Kaiser, which went up 23% last years,” White said.

The company said refinery operations will continue despite the strike, and it does not anticipate any supply chain issues. If the strike were to shut down the refinery, that could negatively affect gasoline prices in California — which has the highest regular gas price in the nation at $5.86 per gallon, according to the American Automobile Association.

The old contract with Chevron in Richmond expired Feb. 1, and workers had been reporting to their jobs on a rolling 24-hour extension, the union said.

The San Ramon, California-based Chevron said on its website that the refinery employs 1,300 workers and produces gasoline, diesel and jet fuel and lubricating oils.

The union represents about 600 employees who include machinists, pipefitters, lab technicians and warehouse workers, White said.

Chevron said in a statement it is “fully prepared to continue normal operations.”

“We anticipate no issues in maintaining a reliable supply of products to the market,” it said.