An oil pipeline rupture near Santa Barbara in California is believed to have spilled up to 2,500 barrels of crude petroleum.
Two oil slicks, stretching nine miles, have formed off the coast of southern California after the onshore pipeline ruptured.
Earlier estimates said a single slick had formed, stretching only four miles.
Officials said the pipeline was running at full-capacity when it broke on Tuesday.
While the pipeline has been switched off, it is unclear how long the cleanup will take.
Shovel and rake wielding workers have been fanning out across the beach to clean up oil that is stuck to sand and rocks along Refugio State Beach, which is on the southern Santa Barbara County coast.
Darren Palmer, an official with Plains All American Pipeline LP, which owns the pipe, said it was running at a rate of about 84,000 gallons an hour when it broke.
The pipe was built in 1991 and had been tested a few weeks ago, he said.
Mr Palmer said the company takes responsibility for the spill and will pay for the cleanup.
Environmentalists are concerned that the oil may harm wildlife including birds and whales.
"It smells like what they use to pave the roads," Fan Yang told the Associated Press news agency. "I'm sad for the birds, if they lose their habitat".
The spill happened on the same stretch of beach as a 1969 spill - one that is credited with starting the American environmental movement. In that incident, several hundred thousand gallons spewed from a blowout on an oil platform, ultimately resulting in the death of thousands of seabirds and many marine mammals.
The alarm was first raised on Tuesday when authorities received reports of a foul smell near Refugio State Beach around midday local time.
They traced the oil to a ruptured onshore pipeline that was spewing oil into a rain water drain which ran to the ocean. The pipeline was shut off about three hours later.
The area has been closed to recreational and fishing activities. It is not clear how long it will take to clean up the slicks, or whether the area will be open for the coming Memorial Day weekend - a major holiday in the United States.