A 4-mile stretch of beach in south Los Angeles County was closed Thursday after tar balls washed ashore—the latest Southern California coastline to shut down due to oily goo, authorities said.

The Long Beach Fire Department ordered the closure Wednesday night over concerns the material could cause skin irritation or other adverse health effects.

"We ask that people stay off the beach and out of the water for now," agency spokesman Jake Heflin said.

By Thursday morning, crews scooped up about 55 gallons of the sticky substance from the shoreline about 25 miles south of downtown Los Angeles, Heflin said. Tar was not readily apparent on the sand at midmorning, between low tide and high tides.

The U.S. Coast Guard will test samples to try to determine the source of the tar balls. The material did not appear related to oil and gas extraction operations in the area, Heflin said.

The beach is adjacent to the massive Long Beach-Los Angeles port complex.

Tar balls also led to recent beach closures in nearby Manhattan Beach and farther northwest in Ventura County.

Nothing has been ruled out, including last month's spill of thousands of gallons of crude on the Santa Barbara County coast, about 140 miles to the northwest. Two beaches there remain closed.

The City of Seal Beach also reported an increase Thursday in the number of tar balls found on the beach. The Coast Guard determined there was no need to close the beach and suggested letting the tar balls dissipate naturally, city officials said.

Public works employees will pick up the tar balls during daily beach cleaning operations.