A string of popular California beaches will remain closed until further notice due to mystery oil-based pollution, officials said Friday.

The stretch of coastline south of Los Angeles was closed to the public from Wednesday, barely a week after an oil spill 100 miles further north, although there is still no confirmed link between the two.

On Thursday officials said clean-up workers had made "significant progress," leading to speculation that they could re-open as early Friday. But authorities downplayed that.

"The beaches are closed until we make a determination that it's safe," said US Coast Guard spokeswoman Charlene Downey, referring to the stretch including the El Segundo, Manhattan, Hermosa and Redondo beaches.

The unidentified pollution comes after thousands of gallons of oil spilled into the ocean, some from a ruptured pipe near Santa Barbara, some 100 miles (160 kilometers) northwest up the coast.

In last week's oil spill up to 105,000 gallons leaked from a crude pipeline. Two beaches were closed, and remain so until June 4, officials said Thursday. Fishing rights were also suspended within a 30-kilometer by 10-kilometer area.

About 10 seals and sea lions covered in oil were rescued, while dead animals included lobsters and pelicans.

Santa Barbara was the scene of what was then the largest oil spill in US history, when in 1969 several million gallons of crude spewed into the ocean after an oil rig blowout.