Conservative U.S. Supreme Court justices on Monday appeared skeptical of the federal government's ability to issue sweeping regulations to reduce carbon emissions from power plants in a case that could undermine President Joe Biden's plan to tackle climate change.

The court, whose 6-3 conservative majority has shown wariness toward broad federal agency actions, was weighing the Environmental Protection Agency's (EPA) authority to regulate greenhouse gas emissions from existing coal- and gas-fired power plants under the landmark Clean Air Act.

Although some justices questioned the EPA's power in an abstract sense, it remained unclear how the court would rule, as lawyers for the EPA and power companies pushed back against a decision that would prevent the agency from issuing any regulation that went beyond restrictions on individual plants.

An eventual ruling restricting the EPA's authority could hamstring the administration's ability to curb the power sector's emissions - representing about a quarter of U.S. greenhouse gases. The United States, behind only China in greenhouse gas emissions, is a pivotal player in efforts to combat climate change on a global basis.

Justice Samuel Alito was one of the conservative justices who seemed to believe that any broad assertion of authority sought by the EPA would constitute a "major question" that under court precedent requires Congress to have expressly authorized it.

Alito told Solicitor General Elizabeth Prelogar, the Biden administration's lawyer, that the agency does not just want to regulate individual plants but instead wants to "set energy policy" for the whole country.

The Supreme Court is reviewing the U.S. Court of Appeals for the District of Columbia Circuit's 2021 decision striking down Republican former President Donald Trump's Affordable Clean Energy rule. That regulation would have imposed limits on a Clean Air Act provision called Section 111 that provides the EPA authority to regulate emissions from existing power plants.

The argument was held hours after the release of a 3,675-page United Nations report urging action on a global scale to combat climate change. Outside the Supreme Court, a small group of demonstrators carried signs reading "Protect the Clean Air Act."

The case was pursued by Republican-led states led by coal producer West Virginia. Other challengers include coal companies and coal-friendly industry groups. Coal is among the most greenhouse gas-intensive fuels.

Democratic-led states and major power companies including Consolidated Edison Inc, Exelon Corp and PG&E Corp sided with Biden's administration, as did the Edison Electric Institute, an investor-owned utility trade group.

The rule proposed by Trump, a supporter of the U.S. coal industry who also questioned climate change science, was meant to supplant Democratic former President Barack Obama's Clean Power Plan mandating major reductions in carbon emissions from the power industry.

The Supreme Court blocked implementation of the Clean Power Plan in 2016 without ruling on its lawfulness.

Coal-aligned groups now want the justices to rule that Biden's administration cannot take a sweeping approach to regulating carbon emissions under Section 111. Such a decision would prevent the EPA from enforcing industry-wide changes, limiting it to actions targeting individual plants.

That would be a blow for Biden's administration, which has a goal of decarbonizing the U.S. power sector by 2035. The White House's incentive-based proposal to achieve that goal was rejected in Congress during budget and infrastructure legislation negotiations.

The Supreme Court already has shown antagonism toward broad agency actions and has relied on the "major questions" doctrine, most recently on Jan. 13 by blocking Biden's COVID-19 vaccine-or-test mandate for large employers.

If Biden's administration loses the case, Congress would need to pass new legislation for the government to impose sweeping climate-related regulations - an unlikely prospect in the near-term given stubborn divisions among lawmakers.

Prelogar said the EPA will unveil a proposed new regulation by the end of the year, which would likely come after the Supreme Court's ruling that is expected by the end of June.