California officials were trying Sunday to limit damage from a major oil spill off the coast of Orange County, as oil reached beaches and threatened wetlands south of Los Angeles.

The spill of an estimated 126,000 gallons from an oil-processing platform about 5 miles offshore extended from Newport Beach to Huntington Beach, a distance of roughly 6 miles.

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California officials were trying Sunday to limit damage from a major oil spill off the coast of Orange County, as oil reached beaches and threatened wetlands south of Los Angeles.

The spill of an estimated 126,000 gallons from an oil-processing platform about 5 miles offshore extended from Newport Beach to Huntington Beach, a distance of roughly 6 miles.

Martyn Willsher, chief executive of Houston-based Amplify Energy Corp. , which operates the oil platform, said the pipeline has been shut off and any remaining oil in it had been suctioned off, preventing further releases.

The company notified the Coast Guard about the spill Saturday morning after workers conducting a line inspection noticed a sheen in the water, Mr. Willsher said. The offshore platform receives oil from about 70 wells and then sends it to a local refinery.

Huntington Beach was one of the areas affected by the oil spill off the California coast.

Photo: Mario Tama/Getty Images

He said the company’s operations were shut down as divers were trying to find the source and cause of the spill in a pipeline about 80 to 100 feet underwater. “We will do everything in our power to ensure that this is recovered as quickly as possible,” he said.

Rep. Michelle Steel sent a letter Sunday to President Biden requesting a major disaster declaration for Orange County.

Local health officials told residents in the affected area to avoid walking, swimming or surfing near the affected beaches and wetlands. They also advised people to avoid touching any oil on beaches or attempting to rescue any wildlife affected by the oil.

The oil spread into Talbert Marsh, which is home to many bird species.

Photo: Mario Tama/Getty Images

Capt. Rebecca Ore of the Coast Guard said teams were working both offshore and on land to recover as much of the crude oil that spilled to minimize the potential impact. The Coast Guard previously said the spill covered a 13-square-mile area off the coast.

“We have a large sheen that is approaching our pristine California beaches, which I know is very upsetting to the residents of California,” she said.

Orange County Supervisor Katrina Foley said that when she visited a local beach Sunday she saw clusters of oil on the shoreline and that she could smell vapors from the spill in the air.

“Please don’t go to the beach and try to help,” she said. “We are dealing with something toxic.”

Other state and local officials echoed her warning. Dr. Clayton Chau, director of the Orange County Health Care Agency, said the county planned to issue a health advisory related to the spill.

Lifeguards prepared signs warning the public to stay off beaches after the oil spill.

Photo: christian monterrosa/Shutterstock

He advised residents to seek medical attention if they experienced effects from vapors such as irritation to their eyes and throat or dizziness and vomiting.

Huntington Beach officials had canceled the third and final day of the Pacific Airshow that had been scheduled to take place Sunday, because of the spill. Mayor Kim Carr said Saturday that the spill could cause a “potential ecological disaster.”

Booms and other equipment were deployed on Saturday to prevent oil from reaching sensitive wetlands and nature reserves.

On Sunday, there were reports of dead fish and birds washing ashore near Newport Beach and Huntington Beach, according to Ms. Foley, the county supervisor. At a press briefing, a spokesman for the California Department of Fish and Wildlife said he was aware of one confirmed report of a duck that had been affected by the oil. It was being treated by a veterinarian.

Write to Kris Maher at kris.maher@wsj.com