Republican leadership is blaming Democratic US President Joe Biden for the huge oil spill off California's southern coast that may have been caused by a containership anchor.

US authorities suspect that an anchor to one of dozens of boxships in San Pedro Bay may have torn an underwater Amplify Energy oil pipeline on 1 October, causing the release of at least 126,000 gallons of oil.

The US Coast Guard is still investigating how the pipeline received a 33-centimetre gash and was moved 32 metres.

Global supply-chain disruption has left long queues of containerships and bulkers waiting outside ports worldwide for an open berth to discharge their cargoes.

As of Wednesday, 57 containerships were anchored in the bay, awaiting available berths at the ports of Los Angeles and Long Beach, according to Port of Los Angeles data.

Operating 24 hours a day

Biden announced on Wednesday that both ports have started to operate non-stop to ease containership congestion, weeks after Port of Los Angeles officials asked for White House help in making that happen.

Republican congressmen Sam Graves representing Missouri and Bruce Westerman representing Arizona are, however, still putting the onus for the huge spill on Biden.

Graves is a ranking member of the US House of Representatives Committee on Transportation and Infrastructure. Westerman is a ranking member of the House Committee on Natural Resources.

"The fact that this oil spill likely occurred due to the growing number of containerships bottled up offshore demonstrates how the entire infrastructure network is interconnected and why we have to focus on the functionality of this core system," Graves said in a statement.

"Its causes are rooted in the ongoing supply chain issues that this administration has done little to address."

Westerman said the containership congestion was a "direct threat to critical underwater infrastructure" along California's coastline.

"It would be laughable if it wasn't such a serious issue, one that has already had massive economic implications and is now having massive environmental implications," he said.

"Until President Biden and congressional Democrats look at the facts and take responsibility for this bottleneck and the subsequent oil spill, we're going to keep finding ourselves in the same tragic situation."