It was unclear whether the tar balls, which began washing up around Long Beach on Wednesday, had any link to an oil spill last month or another mystery pollution incident up the coast last week.
The latest stretch of affected coastline will remain closed until health and environmental concerns can be addressed, Long Beach Fire Department spokesman Jake Heflin said.
"There have been ongoing efforts ... to make sure we are mitigating and minimizing any potential impact, not only on our beaches ... but also protecting and preserving public health and public safety," he told reporters.
On May 19, an oil pipeline ruptured near Santa Barbara, some 110 miles (175 kilometers) northwest of Los Angeles, spilling thousands of gallons of crude into the ocean and closing at least two beaches.
Then last week, tar balls suddenly began washing ashore from El Segundo Beach down to Redondo Beach, about 20 miles (32 km) southwest of downtown Los Angeles, forcing the closure of four local beaches, AFP reported.
Long Beach, about another 10 miles (16 km) southeast of Redondo Beach and 25 miles (40 km) south of downtown Los Angeles, is home to the biggest industrial port on the US West Coast.