A fleet of 18 vessels is battling to contain an oil slick stretching nine miles along the California coast as rescuers on land try to save seals, birds and other wildlife trapped in the black goo.

Crews deployed more than 3,000 feet of boom on Thursday to corral some of the oil which flushed into the Pacific ocean after a pipeline broke on Tuesday, prompting a protest rally and renewed calls for stronger safeguards on oil transport.

'Nightmare' California oil spill damages rare coastal ecosystem

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Up to 105,000 gallons may have leaked from the ruptured pipeline, with about 21,000 of those gallons reaching the once-pristine waters off Santa Barbara, according to estimates.

California's governor, Jerry Brown, has declared a state of emergency in Santa Barbara county to cut red tape and facilitate the mobilisation of resources.

The Houston-based company which runs the underground pipeline, Plains All American Pipeline, said on Thursday the 24-inch pipe was carrying 1,300 barrels an hour, below its maximum capacity of 2,000 barrels an hour, when the leak was detected on Tuesday morning.