The Port of Los Angeles has said an oil spill from a subsea pipeline off the coast of California will not impact containership operations at the US west coast port.

On Sunday, the US Coast Guard (USCG) said it was responding to an oil sheen approximately 6 km (four miles) off the coast in Southern California.

"While some vessels waiting to come into San Pedro Bay may need to be cleaned, Port of Los Angeles operations and arrivals have NOT been impacted," the port said on social media.

"All Port of Los Angeles terminals are open and operational, with 25 vessels in port today. Labour crews are working on 18 containerships, one tanker and three dry bulk carriers."

US authorities have said a ship’s anchor may be to blame for the incident that has been described as one of California’s largest oil spills.

The head of the company that operates the pipeline said that divers have examined more than 8,000 feet of pipe and are focusing on "one area of significant interest".

An anchor striking the pipeline is "one of the distinct possibilities" behind the leak, Amplify Energy chief executive Martyn Willsher told a news conference.

"This response is currently a 24/7 operation and response efforts are scheduled to continue until federal and state officials determine that the response to the crude oil spill is complete," the USCG said of the clean-up operation.

"As of Sunday, a total of 1,218 gallons of oily water mixture have been recovered, nine boats were dispatched for oil spill recovery operations, three shoreline assessment teams have been dispatched and 3,700 feet of boom have been deployed."

Port congestion, particularly at Los Angeles and Long Beach, continues to exacerbate as they struggle to cope with strong US imports, according to US investment JP Morgan.

"There appears to be a lack of options for liners to unload cargo, with some attempting to divert to other US west coast ports," the bank said in a recent report.

Waiting times at the two major US east coast ports have seen 70 or more containerships anchored in San Pedro Bay.