The third challenge to the landmark law, formally known as the Affordable Care Act, comes as the United States records some of its highest coronavirus infection rates since the contagion hit the country.

Under Obamacare, millions of Americans are required to buy health insurance or face a tax penalty.

But in 2017 Congress eliminated the fine for people who failed to sign up -- known as the individual mandate -- removing a key part of former president Barack Obama's policy.

The Department of Justice (DoJ) argues "the individual mandate is not severable from the rest of the Act."

Because of that "the mandate is now unconstitutional as a result of Congress's elimination ... of the penalty for noncompliance," it said in a late filing.

As a result "the entire ACA thus must fall with the individual mandate."

The DoJ also argues that ACA coverage protecting people with pre-existing conditions -- rules that mean insurers cannot refuse customers because of their age, gender or health status -- should also be overturned.

The Supreme Court will hear the case in its next term starting October, but US media reported that it is unlikely to be examined before the presidential election in November. 

Democrat House Speaker Nancy Pelosi condemned the Trump administration's move and called it an "act of unfathomable cruelty" during the pandemic.

She claimed if passed 130 million Americans with pre-existing conditions could lose the ACA's protections, and as many as 23 million citizens could be left without any insurance.

"There is no legal justification and no moral excuse for the Trump Administration's disastrous efforts to take away Americans' health care," she said.

The US has been particularly badly hit by the coronavirus pandemic -- and unlike Europe and parts of East Asia, has never climbed down from its peak.

Twenty-nine states are now experiencing fresh surges, with almost 40,000 new cases recorded, according to the Centers for Disease Control and Prevention.

US health officials now believe based on antibody surveys that about 24 million people may have been infected at some point -- 10 times higher than the officially recorded figure of around 2.4 million.