Popular beaches along nearly seven miles (11 kilometres) of Los Angeles-area coastline were off-limits to surfing and swimming on Thursday as scientists looked for the source of globs of tar that washed ashore.

The sand and surf on south Santa Monica bay appeared virtually free of oil after an overnight clean-up, but officials weren’t sure if more tar would show up. They planned to assess during low tide at midday.

Public health officials told people to avoid contact with the water, wet sand or any material that washed up in the area. They warned that contact with petroleum products can cause skin irritation and result in long-term health problems.

Lifeguards chased a handful of surfers out of the water, but beach life was otherwise normal for people exercising, playing volleyball, skating and riding bikes along the shore.

US Coast Guard and state officials said samples of tar and water would be analysed to identify where it originated, but it could take days to get the results. Nothing has been ruled out, including last week’s coastal oil spill that created a 10-square-mile (26-sq km) slick about 100 miles (160km) to the north-west, off the Santa Barbara County coast.

There is also a refinery and offshore oil tanker terminal nearby, but the coast guard did not find a sheen from a spill after the tar started to accumulate on Wednesday.

No problems with wildlife have been reported, said Sau Garcia from the California Department of Fish and Wildlife.