WASHINGTON – US Supreme Court justices on Tuesday appeared inclined to shield a unit of British oil major BP from claims seeking a more extensive cleanup of a Superfund hazardous waste site in Montana than what federal environmental officials had ordered.

The nine justices heard arguments in an appeal by Atlantic Richfield of a Montana state court ruling allowing a group of private landowners within the sprawling site of its former Anaconda copper smelter in western Montana to bring their claims for restoration damages to trial.

Liberal and conservative justices alike signaled their concern that landowners could interfere with land remediation efforts ordered by the US Environmental Protection Agency.

Atlantic Richfield said the lower court's decision could lead to the filing of thousands more lawsuits against companies nationwide, and further complicate federally mandated improvements to contaminated land.

Atlantic Richfield, backed by President Donald Trump's administration and industry lobby groups including the US Chamber of Commerce and the National Association of Manufacturers, has spent $450-million on soil and ground water restoration at the site ordered by the EPA.

The Superfund program, started in 1980, is intended to identify contaminated sites and ensure that those responsible for the pollution pay for the hazardous waste cleanup. It has been criticized over the years for slow efforts.

The case hinges on the scope of the Superfund law, called the Comprehensive Environmental Response, Compensation and Liability Act.

The Anaconda smelter, near the small community of Opportunity, Montana, operated between 1884 and 1980 and provided much of the world's copper supply.

The area is filled with creeks and streams that cross forests and farmland. It was designated as a Superfund site in 1983 to reduce arsenic contamination in residential yards, pastures and ground water.

Conservative Chief Justice John Roberts said that there could be "significant adverse impacts" from landowners taking actions that the EPA would oppose, noting the agency's concerns about ground water in the Montana case.

Liberal Justice Stephen Breyer said channeling efforts through the EPA would avoid "10 000 juries or 50 states" imposing conflicting duties.

Much of the argument focused on whether each landowner was considered a "potentially responsible party" who must seek the EPA's approval under the law before undertaking restoration of their own contaminated land.

The case began in 2008 when the landowners sued in state court to restore their properties to pre-smelter conditions.

Atlantic Richfield said such state law claims were barred by the EPA's actions under the Superfund law. The company also said the litigation was prohibited by the US Constitution's so-called supremacy clause, which holds that federal law generally trumps state law.