An oil slick is seen along the coast of Refugio State Beach in Goleta, California, United States, May 19, 2015. [Photo/Agencies]

The Coast Guard, county emergency officials and state parks officials were cleaning up the spill. Boats from the nonprofit collective Clean Seas also were providing help but were having trouble because so much of the oil was so close to the shore, Coast Guard spokeswoman Jennifer Williams said. About 850 gallons (3,200 liters) of oil have been recovered from the water, Williams said.

The accident occurred on the same stretch of coastline as a 1969 spill that at the time was the largest ever in US waters and is credited for giving rise to the American environmental movement. Several hundred thousand gallons spilled from a blowout on an oil platform and thousands of sea birds were killed along with many marine mammals. It was later surpassed in size by 1989's Exxon Valdez spill off Alaska and the 2010 Gulf oil spill off Louisiana.

The stretch of coastline about 20 miles (32 kilometers) northwest of the pricey real estate of Santa Barbara still is home to offshore oil rigs and small amounts of tar and seepage regularly show up on beaches.

The spill is largest in years and the Santa Barbara-based Environmental Defense Center said to have it occur in "a sensitive and treasured environment is devastating to watch.'' The group expressed special worry for the many species of whale that migrate through the area.

It was unclear how long the cleanup would take and whether Refugio and other areas would be reopened in time for the Memorial Day holiday weekend.

Offshore cleanup was suspended for the night, and the Coast Guard said a morning flyover would allow them to get a better sense of the damage.

Overnight winds are expected to push the spill down the coast and closer to Santa Barbara.