Oil from a ruptured pipeline in southern California leaked into the Pacific Ocean on Tuesday, spreading into a four-mile-long slick and fouling a Santa Barbara County beach.

The cause of the release and the exact amount spilled were still unknown Tuesday evening.

An initial report from the Governor's Office of Emergency Services estimated that more than 500 barrels, or 21,000 gallons, were released. But the true volume of oil spilled is still being measured, said Capt. Jennifer Williams, the federal on-scene coordinator with the U.S. Coast Guard.

The Coast Guard is monitoring winds in the area and projects that overnight the spill may move two to four miles south of Refugio State Beach in Santa Barbara County.

At first light, the Coast Guard will fly over by helicopter to assess the oil sheen in the water and impact to beaches, Capt. Williams said, adding that this spill appeared to be "medium"-size.

The oil spilled from a large pipeline that runs under U.S. Highway 101, according to the pipeline's owner, Plains All American Pipeline L.P. The pipeline, which can carry up to 150,000 barrels of oil a day, transports crude from an inland facility inland owned by Exxon Mobil Corp.

"The spill has impacted ocean water and the shoreline," the company said in a statement released late Tuesday. "Plains deeply regrets this release has occurred and is making every effort to limit its environmental impact."

Exxon Mobil offered to assist with clean-up efforts on Tuesday but was told its aid was not yet needed, a company spokesman told the Wall Street Journal.

Plains shut down the pipeline to stop the flow of oil and the culvert where the leak was discovered has been blocked so no additional crude can reach the water, the company said.

Officials from the Santa Barbara Fire Department evacuated people from Refugio State Beach after the spill was reported around 1 p.m. local time. Refugio State Beach is situated on a cove teeming with wildlife about 140 miles northwest of Los Angeles.

No injuries to humans have been reported, Plains said. So far, there have been no reports of injured or oiled birds or other wildlife, said Santos Cabral of the California Fish and Game Department.

Clean-up efforts started Tuesday, with 1,500 feet of boom—a kind of floating curtain—deployed on the water to trap oil floating at the surface. So far, only 20 barrels of oil have been skimmed off the water, according to the Coast Guard.

"I want to manage your expectations. This could take some time," Capt. Williams said, adding that the unified joint command responding to the spill does not intend to use chemical dispersants to dissolve the oil in the water.

The spill was discovered after people noticed a strong petroleum odor. Oil was found on the beach and in the water, with early reports of a sheen about a quarter-mile long, according to the report from the governor's office. By late afternoon, the sheen had grown to nearly four miles, according to Coast Guard officials who flew over the site.

"This is going to be a major cleanup effort," said Santa Barbara County Fire Department Capt. Dave Zaniboni.

The beach is one of the first stretches of undeveloped shoreline heading north out of Los Angeles. The protected cove is prized by surfers, kayakers, campers and fishing enthusiasts—as well as seals, sea lions and birds.

"It's our workplace, and our playground and our passion," said Grant Cunningham, general manager for Santa Barbara Adventure Company, which runs kayaking trips and hikes in the area. "To see it covered in covered in oil is devastating."

Santa Barbara was the site of a January 1969 oil spill that helped spark the modern environmental movement. In that accident, a Union Oil Co. well at an offshore platform suffered a blowout and oil began leaking into the Santa Barbara channel. Within a week, the oil washed ashore, fouling beaches and wildlife.

A little more than a year after the Santa Barbara spill the first Earth Day was held in the U.S. By the end of 1970. Then-President Richard Nixon halted any further growth in the oil industry's offshore activity in the state, imposing a ban on drilling off the coast of California that is still in effect today.

"Big Oil comes with big risks—from drilling to delivery. Santa Barbara learned that hard lesson over 40 years ago when offshore drilling led to disaster," said Bob Deans, a spokesman for the Natural Resources Defense Council. "It's time to invest in clean energy. We need real solutions that don't spill, explode or cause climate chaos."

This isn't the first Plains pipeline leak in the state. Before Tuesday's leak there had been four oil spills of 100 barrels or more from the company's California pipelines since 2010, according to the U.S. Pipeline and Hazardous Materials Safety Administration.

Last year about 330 barrels, or 14,000 gallons, leaked from a Plains pipe in an industrial area of Los Angeles. That spill was caused by a malfunctioning valve. Firefighters responding to the incident were confronted with a 20-to-50-foot stream of oil that sprayed against the side of a strip club.

Russell Gold contributed to this article.

Write to Tamara Audi at tammy.audi@wsj.com and Alison Sider at alison.sider@wsj.com

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