A slick of unidentified tar-like substance forced the closure of another stretch of the California coast on Wednesday as federal authorities ordered the company at the center of a spill last week to continue its efforts to clean up after the pipeline breach that poured crude oil onto a stretch of pristine shore.

Los Angeles County lifeguards spotted mysterious blobs washed up on Manhattan Beach at 10 a.m. and the substance continued to wash ashore on the surf for about two hours, according to the Los Angeles County Fire Department.

The globs fouled a 6.8-mile stretch, according to according to ABC 7, and authorities closed the beaches as precaution.

Coast Guard petty officer Marshal Anderson described the small balls as oil patties to NBC Los Angeles.

The fire department said investigators have collected samples and are examining the cause, composition, and source of the unknown substance.

Manhattan Beach Mayor Wayne Powell told the Beach Reporter that the substance washing ashore does not appear to be coming from a natural source.

“Right now it doesn't appear to be a big oil spill, but the source is unknown,” Powell told the Reporter. “Periodically oil washes up naturally, but that is not the case here today. There are already people here suited up beginning to clean it up.”

The unusual oil slick in Manhattan Beach comes after a May 19 spill some 100 miles north that dumped more than 100,000 gallons of crude oil onto the Santa Barbara coastline and into the Pacific, leaving slicks that stretched 9 miles along the coast.

In ordering Plains All American Pipeline to submit a written plan by June 6 that will outline measures for analyzing the spills effects on the environment, Jared Blumenfeld of the U.S. Environmental Protection Agency said the federal government wanted “to make sure the oil response work continues until the Santa Barbara County coastline is restored.”

Coast Guard Captain Jennifer Williams, the federal on-scene coordinator for the response team handling the Santa Barbara spill, said in a statement the order defines Plains All American as the “responsible party” and that government agencies would continue to work alongside the company on the cleanup.

A representative for Plains All American could not be reached for comment.

About 1,000 workers, including federal and state employees, are taking part in cleanup work. Two state beaches were closed indefinitely, along with fishing in the area where Gov. Jerry Brown declared a state of emergency.

Also on Wednesday, the conservation group the Center for Biological Diversity urged California regulators to reject a proposed expansion of the only offshore drilling operation still permitted in state waters along the Santa Barbara coastline.

Privately owned Venoco Inc. is seeking permission to drill on 3,400 acres of the sea floor within a state-designated coastal sanctuary adjacent to the company's current offshore lease site. It said the plan would increase petroleum production by 6,400 barrels a day.

Al Jazeera with wire services