Officials have told residents of two Northern California cities to shelter-in-place as a fire at a Chevron refinery in Richmond releases plumes of black smoke.

The fire, first reported at 6:40 p.m. Monday, is burning in a process unit at the refinery, officials said. It is sending smoke over the cities of Richmond and San Pablo.

Residents are being advised by officials with Contra Costa County Health Services to turn off heaters, air conditioners and fans, and to cover cracks around doors with tape or damp towels.

Refinery spokeswoman Heather Cox told KTVU-TV that there were no injuries to report. Cox said she had no details as to what caused the fire.

Television shots show large flames with thick black smoke coming out of at least two refinery stacks at the facility.

The Chevron Richmond Refinery is among the country’s largest and most important refineries, processing up to 240,000 barrels of crude oil a day, according to the company’s website.

The facility makes high-quality products that include gasoline, jet fuel, diesel fuel and lubricants, as well as chemicals used to manufacture many other useful products. It is the largest producer of base oils on the West Coast.