RESCUE: A worker trying to catch and rehabilitate a bird soaked in oil that was spilt by a ship into the San Francisco Bay. -- PHOTO: AP

A PRELIMINARY investigation has found that human error caused a cargo ship to crash into the Bay Bridge, leading to San Francisco Bay's worst oil spill in nearly two decades, said the US Coast Guard.

'There were skilled enough individuals on board this ship. They didn't carry out their missions correctly,' said Rear Admiral Craig Bone, the Coast Guard's top official in California.

The Cosco Busan was headed out of the bay on Wednesday morning when it sideswiped a support on the San Francisco- Oakland Bay Bridge, rupturing two tanks that leaked about 220,000 litres of heavy fuel into the bay.

Investigators are focusing on possible communication problems among the ship's Chinese crew, its pilot and the Vessel Traffic Service, the Coast Guard station that monitors the bay's shipping traffic.

'The mere fact that they collided with a fixed object' offered clear evidence that a communication problem had occurred, said Coast Guard Commander Andrew Wood.

Petty Officer Sherri Eng said that on Saturday, the Coast Guard increased to 20 the number of ships skimming the oil from the bay, up from 11 the previous day.

According to figures released by the Coast Guard, nearly 75,700 litres of oily liquid had been sucked up by Saturday morning.

But the authorities said most of the spilled oil would never be captured and would eventually dissolve into the water.

Concentrated globules could remain for months and cause problems for seabirds, and rescuers are racing to save hundreds of them.

At least 60 birds have been found dead, while 200 live ones have been recovered and sent to a rehabilitation centre in Solano County.

State wildlife officials said they had received hundreds of reports of oiled birds found on Bay area beaches.

'Oil and feathers don't mix,' said Ms Yvonne Addassi, a wildlife biologist with the state Department of Fish and Game.

'We're in a time crunch. The birds can only stayed oiled for so long before they can no longer be rehabilitated.'

Fish and Game officials said they had not seen any evidence that fish have been harmed, though they are concerned the spill could affect bay herring that spawn at this time of year.

ASSOCIATED PRESS