HUNTINGTON BEACH, CALIFORNIA (REUTERS) - Federal and state investigators on Monday (Oct 4) were probing what caused some 126,000 barrels of crude oil to leak into the Pacific Ocean off the coast of southern California.

Huntington Beach, about 65km south of Los Angeles, was hit hardest, with some 34 sq km of ocean and portions of its coastline "covered in oil", said Mayor Kim Carr.

The oil appeared to come from a production platform operated by Beta Offshore, a California subsidiary of Houston-based offshore crude oil producer Amplify Energy Corporation.

Amplify on Monday did not immediately return a call seeking information.

Federal officials have stepped up their scrutiny of ageing and idled offshore energy pipelines.

Energy companies have built 64,000km of oil and gas pipelines in federal offshore waters since the 1940s.

Regulators have failed to address the risks from idled pipelines, platforms and other infrastructure on the sea floor, the watchdog US Government Accountability Office (GAO) said this year.

"As pipelines age, they are more susceptible to damage from corrosion, mudslides and sea floor erosion," GAO said.

Mr Martyn Willsher, an Amplify executive, said on Sunday a pipeline carrying oil from the platform had been shut off and its remaining oil suctioned out. Divers were trying to determine where and why the spill occurred, he said.

Officials deployed 625m of protective booms, which help contain and slow the oil flows, and about 14,320l had been recovered on Sunday, the US Coast Guard said.