The US Supreme Court has rejected a Trump-backed challenge by Republican-led states to former President Barack Obama's healthcare overhaul.

Thursday's ruling did not address the question of whether a key provision in the law was unconstitutional.

By a comfortable 7-2 vote, the justices of the Supreme Court kicked their case to the kerb.

The ruling suggests that while this court is decidedly more conservative than the one that narrowly upheld Obamacare in a highly charged 2012 decision, there's simply nowhere near a majority of justices interested in reversing that ruling, even if they disagreed with it at the time.

This was the third case seeking to dismantle Barack Obama's signature healthcare reform to reach the Supreme Court - and the law has been sustained by a growing majority each time.

While the conservative bent of many lower federal courts means future challenges could still get a toehold in the judicial system, it's increasingly clear that the Supreme Court - the nation's ultimate legal arbiter - has no interest in striking down the law.

Thursday's ruling left in place an Obamacare requirement known as the "individual mandate" - a provision instructing people to buy health insurance or face a tax penalty.

The 18 Republican states involved in the suit argued that the law became unconstitutional once the penalty was eliminated.

The majority opinion was written by Stephen Breyer, the most senior liberal justice on the court.

The ruling comes from the most conservative Supreme Court benches in decades, after former President Trump, a Republican, nominated three justices in his four-year term.

In a tweet, Congressman Kevin Brady, a Texas Republican, said he was "disappointed" by the ruling and criticised the Supreme Court for having "stepped in" to save the "flawed" legislation.

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